Here’s a few notes on CPUID and /proc/cpuinfo. I made a table for quick reference.

What is CPUID?

On x86 and amd64 CPUs, there is a large swathe of differences in features available to the software. Some CPUs ship with AES encryption support, others ship with virtualization support, almost all ship with a collection of SIMD (single instruction multiple data) instructions. The name of the game is to reduce CPU execution times using specialized instructions. Gratefully, most users don’t need to think about this, credit to all the various code-paths found in their computer software; if your CPU doesn’t support particular instructions, a naive, slower implementation can fill in the gaps.

In other cases, one needs to know in order to configure embedded or low end hardware for efficient operation. When software is tightly coupled to a single sort of computer architecture, many efficiencies can be realized at a cost of software portability. For example, I am writing this article on a Pentium M laptop 20 years past its prime, yet runs modern Linux just-the-same. It can even run Firefox, albeit rather slowly. Discord will load! Part of this feat is attributed to optimizing software specifically for the quirks of this computer’s architecture: an ever-stalling instruction pipeline, a minute cache size, and a low memory environment. CPU feature detection via CPUID for legacy hardware is concerned instead with choosing effecient code paths at primarily compile time. Most of these optimizations come at a cost: the software only runs on this specific machine.

There’s another case where it helps to understand CPUID. Persnickety firmwares covet virtualization CPU support like a it’s going out of fashion. A quick glance at CPUID-derived diagnostics is a rite-of-passage for any troubleshooter. If the virtualization feature isn’t in /proc/cpuinfo, then the firmware disabled it, or the hardware simply doesn’t support virtualization.

How do we determine what features are available at runtime or compile time? Enter the CPUID instruction. This instruction enumerates CPU hardware features available to software. From a developer’s perspective, all you need to do is set the eax register (and ecx in some cases!), then execute the CPUID instruction. See OSDev Wiki and Wikipedia for supplementary information. This is how software can detect existence of speedy features like AVX SIMD or AES acceleration at runtime. Utilities for configuring build environments for generating optimized software use CPUID to detect what sort of code to generate for the target architecture.

There are varying ways for looking up the features present on a x86 or amd64 machine and a few Linux techniques are outlined below. The easiest technique is to look for the flags in /proc/cpuinfo — see docs.

The CPUID Table

With hopes to understand each CPUID feature, I looked for chart of terse and minimal content, just enough to understand what’s what. An online search didn’t find such a chart or table. So here is a tabulated form of tools/arch/x86/kcpuid/cpuid.csv (gitweb) as found in the 6.6.30 kernel tree. The table isn’t exhaustive, but everything you care about is likely listed herein.

LEAF SUBLEAF register_name bits short_name long_description
0 0 EAX 31:0 max_basic_leafs Max input value for supported subleafs
1 0 EAX 3:0 stepping Stepping ID
1 0 EAX 7:4 model Model
1 0 EAX 11:8 family Family ID
1 0 EAX 13:12 processor Processor Type
1 0 EAX 19:16 model_ext Extended Model ID
1 0 EAX 27:20 family_ext Extended Family ID
1 0 EBX 7:0 brand Brand Index
1 0 EBX 15:8 clflush_size CLFLUSH line size (value * 8) in bytes
1 0 EBX 23:16 max_cpu_id Maxim number of addressable logic cpu in this package
1 0 EBX 31:24 apic_id Initial APIC ID
1 0 ECX 0 sse3 Streaming SIMD Extensions 3(SSE3)
1 0 ECX 1 pclmulqdq PCLMULQDQ instruction supported
1 0 ECX 2 dtes64 DS area uses 64-bit layout
1 0 ECX 3 mwait MONITOR/MWAIT supported
1 0 ECX 4 ds_cpl CPL Qualified Debug Store which allows for branch message storage qualified by CPL
1 0 ECX 5 vmx Virtual Machine Extensions supported
1 0 ECX 6 smx Safer Mode Extension supported
1 0 ECX 7 eist Enhanced Intel SpeedStep Technology
1 0 ECX 8 tm2 Thermal Monitor 2
1 0 ECX 9 ssse3 Supplemental Streaming SIMD Extensions 3 (SSSE3)
1 0 ECX 10 l1_ctx_id L1 data cache could be set to either adaptive mode or shared mode (check IA32_MISC_ENABLE bit 24 definition)
1 0 ECX 11 sdbg IA32_DEBUG_INTERFACE MSR for silicon debug supported
1 0 ECX 12 fma FMA extensions using YMM state supported
1 0 ECX 13 cmpxchg16b ‘CMPXCHG16B - Compare and Exchange Bytes’ supported
1 0 ECX 14 xtpr_update xTPR Update Control supported
1 0 ECX 15 pdcm Perfmon and Debug Capability present
1 0 ECX 17 pcid Process-Context Identifiers feature present
1 0 ECX 18 dca Prefetching data from a memory mapped device supported
1 0 ECX 19 sse4_1 SSE4.1 feature present
1 0 ECX 20 sse4_2 SSE4.2 feature present
1 0 ECX 21 x2apic x2APIC supported
1 0 ECX 22 movbe MOVBE instruction supported
1 0 ECX 23 popcnt POPCNT instruction supported
1 0 ECX 24 tsc_deadline_timer LAPIC supports one-shot operation using a TSC deadline value
1 0 ECX 25 aesni AESNI instruction supported
1 0 ECX 26 xsave XSAVE/XRSTOR processor extended states (XSETBV/XGETBV/XCR0)
1 0 ECX 27 osxsave OS has set CR4.OSXSAVE bit to enable XSETBV/XGETBV/XCR0
1 0 ECX 28 avx AVX instruction supported
1 0 ECX 29 f16c 16-bit floating-point conversion instruction supported
1 0 ECX 30 rdrand RDRAND instruction supported
1 0 EDX 0 fpu x87 FPU on chip
1 0 EDX 1 vme Virtual-8086 Mode Enhancement
1 0 EDX 2 de Debugging Extensions
1 0 EDX 3 pse Page Size Extensions
1 0 EDX 4 tsc Time Stamp Counter
1 0 EDX 5 msr RDMSR and WRMSR Support
1 0 EDX 6 pae Physical Address Extensions
1 0 EDX 7 mce Machine Check Exception
1 0 EDX 8 cx8 CMPXCHG8B instr
1 0 EDX 9 apic APIC on Chip
1 0 EDX 11 sep SYSENTER and SYSEXIT instrs
1 0 EDX 12 mtrr Memory Type Range Registers
1 0 EDX 13 pge Page Global Bit
1 0 EDX 14 mca Machine Check Architecture
1 0 EDX 15 cmov Conditional Move Instrs
1 0 EDX 16 pat Page Attribute Table
1 0 EDX 17 pse36 36-Bit Page Size Extension
1 0 EDX 18 psn Processor Serial Number
1 0 EDX 19 clflush CLFLUSH instr
1 0 EDX 21 ds Debug Store
1 0 EDX 22 acpi Thermal Monitor and Software Controlled Clock Facilities
1 0 EDX 23 mmx Intel MMX Technology
1 0 EDX 24 fxsr XSAVE and FXRSTOR Instrs
1 0 EDX 25 sse SSE
1 0 EDX 26 sse2 SSE2
1 0 EDX 27 ss Self Snoop
1 0 EDX 28 hit Max APIC IDs
1 0 EDX 29 tm Thermal Monitor
1 0 EDX 31 pbe Pending Break Enable
4 0 EAX 4:0 cache_type Cache type like instr/data or unified
4 0 EAX 7:5 cache_level Cache Level (starts at 1)
4 0 EAX 8 cache_self_init Cache Self Initialization
4 0 EAX 9 fully_associate Fully Associative cache
4 0 EAX 25:14 max_logical_id Max number of addressable IDs for logical processors sharing the cache
4 0 EAX 31:26 max_phy_id Max number of addressable IDs for processors in phy package
4 0 EBX 11:0 cache_linesize Size of a cache line in bytes
4 0 EBX 21:12 cache_partition Physical Line partitions
4 0 EBX 31:22 cache_ways Ways of associativity
4 0 ECX 31:0 cache_sets Number of Sets - 1
4 0 EDX 0 c_wbinvd 1 means WBINVD/INVD is not ganranteed to act upon lower level caches of non-originating threads sharing this cache
4 0 EDX 1 c_incl Whether cache is inclusive of lower cache level
4 0 EDX 2 c_comp_index Complex Cache Indexing
5 0 EAX 15:0 min_mon_size Smallest monitor line size in bytes
5 0 EBX 15:0 max_mon_size Largest monitor line size in bytes
5 0 ECX 0 mwait_ext Enum of Monitor-Mwait extensions supported
5 0 ECX 1 mwait_irq_break Largest monitor line size in bytes
5 0 EDX 3:0 c0_sub_stats Number of C0* sub C-states supported using MWAIT
5 0 EDX 7:4 c1_sub_stats Number of C1* sub C-states supported using MWAIT
5 0 EDX 11:8 c2_sub_stats Number of C2* sub C-states supported using MWAIT
5 0 EDX 15:12 c3_sub_stats Number of C3* sub C-states supported using MWAIT
5 0 EDX 19:16 c4_sub_stats Number of C4* sub C-states supported using MWAIT
5 0 EDX 23:20 c5_sub_stats Number of C5* sub C-states supported using MWAIT
5 0 EDX 27:24 c6_sub_stats Number of C6* sub C-states supported using MWAIT
5 0 EDX 31:28 c7_sub_stats Number of C7* sub C-states supported using MWAIT
6 0 EAX 0 dig_temp Digital temperature sensor supported
6 0 EAX 1 turbo Intel Turbo Boost
6 0 EAX 2 arat Always running APIC timer
6 0 EAX 4 pln Power limit notifications supported
6 0 EAX 5 ecmd Clock modulation duty cycle extension supported
6 0 EAX 6 ptm Package thermal management supported
6 0 EAX 7 hwp HWP base register
6 0 EAX 8 hwp_notify HWP notification
6 0 EAX 9 hwp_act_window HWP activity window
6 0 EAX 10 hwp_energy HWP energy performance preference
6 0 EAX 11 hwp_pkg_req HWP package level request
6 0 EAX 13 hdc HDC base registers supported
6 0 EAX 14 turbo3 Turbo Boost Max 3.0
6 0 EAX 15 hwp_cap Highest Performance change supported
6 0 EAX 16 hwp_peci HWP PECI override is supported
6 0 EAX 17 hwp_flex Flexible HWP is supported
6 0 EAX 18 hwp_fast Fast access mode for the IA32_HWP_REQUEST MSR is supported
6 0 EAX 20 hwp_ignr Ignoring Idle Logical Processor HWP request is supported
6 0 EBX 3:0 therm_irq_thresh Number of Interrupt Thresholds in Digital Thermal Sensor
6 0 ECX 0 aperfmperf Presence of IA32_MPERF and IA32_APERF
6 0 ECX 3 energ_bias Performance-energy bias preference supported
7 0 EBX 1 tsc_adjust TSC_ADJUST MSR supported
7 0 EBX 2 sgx Software Guard Extensions
7 0 EBX 3 bmi1 BMI1
7 0 EBX 4 hle Hardware Lock Elision
7 0 EBX 5 avx2 AVX2
7 0 EBX 7 smep Supervisor-Mode Execution Prevention
7 0 EBX 8 bmi2 BMI2
7 0 EBX 9 rep_movsb Enhanced REP MOVSB/STOSB
7 0 EBX 10 invpcid INVPCID instruction
7 0 EBX 11 rtm Restricted Transactional Memory
7 0 EBX 12 rdt_m Intel RDT Monitoring capability
7 0 EBX 13 depc_fpu_cs_ds Deprecates FPU CS and FPU DS
7 0 EBX 14 mpx Memory Protection Extensions
7 0 EBX 15 rdt_a Intel RDT Allocation capability
7 0 EBX 16 avx512f AVX512 Foundation instr
7 0 EBX 17 avx512dq AVX512 Double and Quadword AVX512 instr
7 0 EBX 18 rdseed RDSEED instr
7 0 EBX 19 adx ADX instr
7 0 EBX 20 smap Supervisor Mode Access Prevention
7 0 EBX 21 avx512ifma AVX512 Integer Fused Multiply Add
7 0 EBX 23 clflushopt CLFLUSHOPT instr
7 0 EBX 24 clwb CLWB instr
7 0 EBX 25 intel_pt Intel Processor Trace instr
7 0 EBX 26 avx512pf Prefetch
7 0 EBX 27 avx512er AVX512 Exponent Reciproca instr
7 0 EBX 28 avx512cd AVX512 Conflict Detection instr
7 0 EBX 29 sha Intel Secure Hash Algorithm Extensions instr
7 0 EBX 30 avx512bw AVX512 Byte & Word instr
7 0 EBX 31 avx512vl AVX512 Vector Length Extentions (VL)
7 0 ECX 0 prefetchwt1 X
7 0 ECX 1 avx512vbmi AVX512 Vector Byte Manipulation Instructions
7 0 ECX 2 umip User-mode Instruction Prevention
7 0 ECX 3 pku Protection Keys for User-mode pages
7 0 ECX 4 ospke CR4 PKE set to enable protection keys
7 0 ECX 21:17 mawau The value of MAWAU used by the BNDLDX and BNDSTX instructions in 64-bit mode
7 0 ECX 22 rdpid RDPID and IA32_TSC_AUX
7 0 ECX 30 sgx_lc SGX Launch Configuration
9 0 ECX 31:0 dca_cap The value of IA32_PLATFORM_DCA_CAP
0xA 0 EAX 7:0 pmu_ver Performance Monitoring Unit version
0xA 0 EAX 15:8 pmu_gp_cnt_num Numer of general-purose PMU counters per logical CPU
0xA 0 EAX 23:16 pmu_cnt_bits Bit wideth of PMU counter
0xA 0 EAX 31:24 pmu_ebx_bits Length of EBX bit vector to enumerate PMU events
0xA 0 EBX 0 pmu_no_core_cycle_evt Core cycle event not available
0xA 0 EBX 1 pmu_no_instr_ret_evt Instruction retired event not available
0xA 0 EBX 2 pmu_no_ref_cycle_evt Reference cycles event not available
0xA 0 EBX 3 pmu_no_llc_ref_evt Last-level cache reference event not available
0xA 0 EBX 4 pmu_no_llc_mis_evt Last-level cache misses event not available
0xA 0 EBX 5 pmu_no_br_instr_ret_evt Branch instruction retired event not available
0xA 0 EBX 6 pmu_no_br_mispredict_evt Branch mispredict retired event not available
0xA 0 ECX 4:0 pmu_fixed_cnt_num Performance Monitoring Unit version
0xA 0 ECX 12:5 pmu_fixed_cnt_bits Numer of PMU counters per logical CPU
0xB 0 EAX 4:0 id_shift Number of bits to shift right on x2APIC ID to get a unique topology ID of the next level type
0xB 0 EBX 15:0 cpu_nr Number of logical processors at this level type
0xB 0 ECX 15:8 lvl_type 0-Invalid 1-SMT 2-Core
0xB 0 EDX 31:0 x2apic_id x2APIC ID the current logical processor
0xD 0 EAX 0 x87 X87 state
0xD 0 EAX 1 sse SSE state
0xD 0 EAX 2 avx AVX state
0xD 0 EAX 4:3 mpx MPX state
0xD 0 EAX 7:5 avx512 AVX-512 state
0xD 0 EAX 9 pkru PKRU state
0xD 0 EBX 31:0 max_sz_xcr0 Maximum size (bytes) required by enabled features in XCR0
0xD 0 ECX 31:0 max_sz_xsave Maximum size (bytes) of the XSAVE/XRSTOR save area
0xD 1 EAX 0 xsaveopt XSAVEOPT available
0xD 1 EAX 1 xsavec XSAVEC and compacted form supported
0xD 1 EAX 2 xgetbv XGETBV supported
0xD 1 EAX 3 xsaves XSAVES/XRSTORS and IA32_XSS supported
0xD 1 EBX 31:0 max_sz_xcr0 Maximum size (bytes) required by enabled features in XCR0
0xD 1 ECX 8 pt PT state
0xD 1 ECX 11 cet_usr CET user state
0xD 1 ECX 12 cet_supv CET supervisor state
0xD 1 ECX 13 hdc HDC state
0xD 1 ECX 16 hwp HWP state
0xF 0 EBX 31:0 rmid_range Maximum range (zero-based) of RMID within this physical processor of all types
0xF 0 EDX 1 l3c_rdt_mon L3 Cache RDT Monitoring supported
0xF 1 ECX 31:0 rmid_range Maximum range (zero-based) of RMID of this types
0xF 1 EDX 0 l3c_ocp_mon L3 Cache occupancy Monitoring supported
0xF 1 EDX 1 l3c_tbw_mon L3 Cache Total Bandwidth Monitoring supported
0xF 1 EDX 2 l3c_lbw_mon L3 Cache Local Bandwidth Monitoring supported
0x10 0 EBX 1 l3c_rdt_alloc L3 Cache Allocation supported
0x10 0 EBX 2 l2c_rdt_alloc L2 Cache Allocation supported
0x10 0 EBX 3 mem_bw_alloc Memory Bandwidth Allocation supported
0x12 0 EAX 0 sgx1 L3 Cache Allocation supported
0x12 1 EAX 0 sgx2 L3 Cache Allocation supported
0x15 0 EAX 31:0 tsc_denominator The denominator of the TSC/”core crystal clock” ratio
0x15 0 EBX 31:0 tsc_numerator The numerator of the TSC/”core crystal clock” ratio
0x15 0 ECX 31:0 nom_freq Nominal frequency of the core crystal clock in Hz
0x16 0 EAX 15:0 cpu_base_freq Processor Base Frequency in MHz
0x16 0 EBX 15:0 cpu_max_freq Maximum Frequency in MHz
0x16 0 ECX 15:0 bus_freq Bus (Reference) Frequency in MHz
0x17 0 EAX 31:0 max_socid Maximum input value of supported sub-leaf
0x17 0 EBX 15:0 soc_vid SOC Vendor ID
0x17 0 EBX 16 std_vid SOC Vendor ID is assigned via an industry standard scheme
0x17 0 ECX 31:0 soc_pid SOC Project ID assigned by vendor
0x17 0 EDX 31:0 soc_sid SOC Stepping ID
0x1A 0 EAX 31:24 core_type 20H-Intel_Atom 40H-Intel_Core
0x80000001 0 EAX 27:20 extfamily Extended family
0x80000001 0 EAX 19:16 extmodel Extended model
0x80000001 0 EAX 11:8 basefamily Description of Family
0x80000001 0 EAX 11:8 basemodel Model numbers vary with product
0x80000001 0 EAX 3:0 stepping Processor stepping (revision) for a specific model
0x80000001 0 EBX 31:28 pkgtype Specifies the package type
0x80000001 0 ECX 0 lahf_lm LAHF/SAHF available in 64-bit mode
0x80000001 0 ECX 1 cmplegacy Core multi-processing legacy mode
0x80000001 0 ECX 2 svm Indicates support for: VMRUN VMLOAD VMSAVE CLGI VMMCALL and INVLPGA
0x80000001 0 ECX 3 extapicspace Extended APIC register space
0x80000001 0 ECX 4 altmovecr8 Indicates support for LOCK MOV CR0 means MOV CR8
0x80000001 0 ECX 5 lzcnt LZCNT
0x80000001 0 ECX 6 sse4a EXTRQ INSERTQ MOVNTSS and MOVNTSD instruction support
0x80000001 0 ECX 7 misalignsse Misaligned SSE Mode
0x80000001 0 ECX 8 prefetchw PREFETCHW
0x80000001 0 ECX 9 osvw OS Visible Work-around support
0x80000001 0 ECX 10 ibs Instruction Based Sampling
0x80000001 0 ECX 11 xop Extended operation support
0x80000001 0 ECX 12 skinit SKINIT and STGI support
0x80000001 0 ECX 13 wdt Watchdog timer support
0x80000001 0 ECX 15 lwp Lightweight profiling support
0x80000001 0 ECX 16 fma4 Four-operand FMA instruction support
0x80000001 0 ECX 17 tce Translation cache extension
0x80000001 0 ECX 22 TopologyExtensions Indicates support for Core::X86::Cpuid::CachePropEax0 and Core::X86::Cpuid::ExtApicId
0x80000001 0 ECX 23 perfctrextcore Indicates support for Core::X86::Msr::PERF_CTL0 - 5 and Core::X86::Msr::PERF_CTR
0x80000001 0 ECX 24 perfctrextdf Indicates support for Core::X86::Msr::DF_PERF_CTL and Core::X86::Msr::DF_PERF_CTR
0x80000001 0 ECX 26 databreakpointextension Indicates data breakpoint support for Core::X86::Msr::DR0_ADDR_MASK Core::X86::Msr::DR1_ADDR_MASK Core::X86::Msr::DR2_ADDR_MASK and Core::X86::Msr::DR3_ADDR_MASK
0x80000001 0 ECX 27 perftsc Performance time-stamp counter supported
0x80000001 0 ECX 28 perfctrextllc Indicates support for L3 performance counter extensions
0x80000001 0 ECX 29 mwaitextended MWAITX and MONITORX capability is supported
0x80000001 0 ECX 30 admskextn Indicates support for address mask extension (to 32 bits and to all 4 DRs) for instruction breakpoints
0x80000001 0 EDX 0 fpu x87 floating point unit on-chip
0x80000001 0 EDX 1 vme Virtual-mode enhancements
0x80000001 0 EDX 2 de Debugging extensions IO breakpoints CR4.DE
0x80000001 0 EDX 3 pse Page-size extensions (4 MB pages)
0x80000001 0 EDX 4 tsc Time stamp counter RDTSC/RDTSCP instructions CR4.TSD
0x80000001 0 EDX 5 msr Model-specific registers (MSRs) with RDMSR and WRMSR instructions
0x80000001 0 EDX 6 pae Physical-address extensions (PAE)
0x80000001 0 EDX 7 mce Machine Check Exception CR4.MCE
0x80000001 0 EDX 8 cmpxchg8b CMPXCHG8B instruction
0x80000001 0 EDX 9 apic advanced programmable interrupt controller (APIC) exists and is enabled
0x80000001 0 EDX 11 sysret SYSCALL/SYSRET supported
0x80000001 0 EDX 12 mtrr Memory-type range registers
0x80000001 0 EDX 13 pge Page global extension CR4.PGE
0x80000001 0 EDX 14 mca Machine check architecture MCG_CAP
0x80000001 0 EDX 15 cmov Conditional move instructions CMOV FCOMI FCMOV
0x80000001 0 EDX 16 pat Page attribute table
0x80000001 0 EDX 17 pse36 Page-size extensions
0x80000001 0 EDX 20 exec_dis Execute Disable Bit available
0x80000001 0 EDX 22 mmxext AMD extensions to MMX instructions
0x80000001 0 EDX 23 mmx MMX instructions
0x80000001 0 EDX 24 fxsr FXSAVE and FXRSTOR instructions
0x80000001 0 EDX 25 ffxsr FXSAVE and FXRSTOR instruction optimizations
0x80000001 0 EDX 26 1gb_page 1GB page supported
0x80000001 0 EDX 27 rdtscp RDTSCP and IA32_TSC_AUX are available
0x80000001 0 EDX 29 lm 64b Architecture supported
0x80000001 0 EDX 30 threednowext AMD extensions to 3DNow! instructions
0x80000001 0 EDX 31 threednow 3DNow! instructions
0x80000006 0 ECX 7:0 clsize Cache Line size in bytes
0x80000006 0 ECX 15:12 l2c_assoc L2 Associativity
0x80000006 0 ECX 31:16 csize Cache size in 1K units
0x80000007 0 EDX 8 nonstop_tsc Invariant TSC available
0x80000008 0 EAX 7:0 phy_adr_bits Physical Address Bits
0x80000008 0 EAX 15:8 lnr_adr_bits Linear Address Bits
0x80000007 0 EBX 9 wbnoinvd WBNOINVD
0x8000001E 0 EAX 31:0 extended_apic_id Extended APIC ID
0x8000001E 0 EBX 7:0 core_id Identifies the logical core ID
0x8000001E 0 EBX 15:8 threads_per_core The number of threads per core is threads_per_core + 1
0x8000001E 0 ECX 7:0 node_id Node ID
0x8000001E 0 ECX 10:8 nodes_per_processor Nodes per processor { 0: 1 node else reserved }
0x8000001F 0 EAX 0 sme Secure Memory Encryption
0x8000001F 0 EAX 1 sev Secure Encrypted Virtualization
0x8000001F 0 EAX 2 vmpgflush VM Page Flush MSR
0x8000001F 0 EAX 3 seves SEV Encrypted State
0x8000001F 0 EBX 5:0 c-bit Page table bit number used to enable memory encryption
0x8000001F 0 EBX 11:6 mem_encrypt_physaddr_width Reduction of physical address space in bits with SME enabled
0x8000001F 0 ECX 31:0 num_encrypted_guests Maximum ASID value that may be used for an SEV-enabled guest
0x8000001F 0 EDX 31:0 minimum_sev_asid Minimum ASID value that must be used for an SEV-enabled SEV-ES-disabled guest

About the columns

the value to set register eax prior to executing the cpuid


the value to set register ecx prior to executing the cpuid instruction. It appears the cpuid instruction looks in ecx only in certain cases. Read the vendor documentation for further explaination.
the register containing the bits after cpuid execution
which range or single bit position to extract from register_name
short symbolic name. Roughly correlates with /proc/cpuinfo
a few words describing the CPUID entry

How did I generate this table?

All in Emacs.

  1. Visit cpuid.csv then copy it. C-x h (mark-whole-buffer), M-w (kill-ring-save).
  2. Visit your target org-mode file.
  3. Paste C-y (yank), Uncomment the commented “key” line that has the column names in it, then move your cursor to the beginning of the first pasted line.
  4. then mark the entire pasted text. C-space (set-mark-command) followed by C-x C-x (exchange-point-and-mark) might work.
  5. Delete unwanted lines M-x flush-lines RET ^#\|^$ RET .
  6. Convert the CSV to an org-mode table C-u C-c | (org-table-create-or-convert-from-region)
  7. That’s it.

Other tables on the net

There are other CPUID tables online, but they didn’t quite meet my needs of compactness.

  • The Wikipedia CPUID article offers a detailed breakdown of all the features
  • The Intel datasheet, AMD datasheet, and additional resources hyperlinked in the aforementioned Wikipedia CPUID article’s References, Further reading, and External links sections.
  • Bunch of others discoverable via Google that are quite detailed.

A few ways to display CPUID feature flags


In the following example see the flags field. This approach requires no external tooling, only a booted Linux kernel. The cinch is one has to correlate the short-names with descriptions or have a brilliant memory.

$ cat /proc/cpuinfo
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 9
model name	: Intel(R) Pentium(R) M processor 1600MHz
stepping	: 5
microcode	: 0x7
cpu MHz		: 1600.000
cache size	: 1024 KB
physical id	: 0
siblings	: 1
core id		: 0
cpu cores	: 1
apicid		: 0
initial apicid	: 0
fdiv_bug	: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 2
wp		: yes
flags		: fpu vme de pse tsc msr mce cx8 sep mtrr pge mca cmov clflush dts acpi mmx fxsr sse sse2 tm pbe bts cpuid est tm2
bugs		: cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds swapgs itlb_multihit mmio_unknown
bogomips	: 3198.33
clflush size	: 64
cache_alignment	: 64
address sizes	: 32 bits physical, 32 bits virtual
power management:


lscpu seems to provide the same information as the above, but with a slimmer output. util-linux provides this package on most distros, so it’s oft installed out of the box.

$ lscpu
Architecture:                       i686
CPU op-mode(s):                     32-bit
Address sizes:                      32 bits physical, 32 bits virtual
Byte Order:                         Little Endian
CPU(s):                             1
On-line CPU(s) list:                0
Vendor ID:                          GenuineIntel
Model name:                         Intel(R) Pentium(R) M processor 1600MHz
CPU family:                         6
Model:                              9
Thread(s) per core:                 1
Core(s) per socket:                 1
Socket(s):                          1
Stepping:                           5
CPU(s) scaling MHz:                 100%
CPU max MHz:                        1600.0000
CPU min MHz:                        600.0000
BogoMIPS:                           3198.33
Flags:                              fpu vme de pse tsc msr mce cx8 sep mtrr pge mca cmov clflush dts acpi mmx fxsr sse sse2 tm pbe bts cpuid est tm2
Vulnerability Gather data sampling: Not affected
Vulnerability Itlb multihit:        KVM: Mitigation: VMX unsupported
Vulnerability L1tf:                 Vulnerable
Vulnerability Mds:                  Vulnerable: Clear CPU buffers attempted, no microcode; SMT disabled
Vulnerability Meltdown:             Vulnerable
Vulnerability Mmio stale data:      Unknown: No mitigations
Vulnerability Retbleed:             Not affected
Vulnerability Spec rstack overflow: Not affected
Vulnerability Spec store bypass:    Vulnerable
Vulnerability Spectre v1:           Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:           Mitigation; Retpolines, STIBP disabled, RSB filling, PBRSB-eIBRS Not affected
Vulnerability Srbds:                Not affected
Vulnerability Tsx async abort:      Not affected


cpuid provides a verbose output with descriptions of all CPUID feature flags. Great if one can install an additional software package available on most distros.

$ cpuid
CPU 0:
   vendor_id = "GenuineIntel"
   version information (1/eax):
      processor type  = primary processor (0)
      family          = 0x6 (6)
      model           = 0x9 (9)
      stepping id     = 0x5 (5)
      extended family = 0x0 (0)
      extended model  = 0x0 (0)
      (family synth)  = 0x6 (6)
      (model synth)   = 0x9 (9)
   miscellaneous (1/ebx):
      process local APIC physical ID = 0x0 (0)
      maximum IDs for CPUs in pkg    = 0x0 (0)
      CLFLUSH line size              = 0x8 (8)
      brand index                    = 0x16 (22)
   brand id = 0x16 (22): Intel Pentium M, .13um
   feature information (1/edx):
      x87 FPU on chip                        = true
      VME: virtual-8086 mode enhancement     = true
      DE: debugging extensions               = true
      PSE: page size extensions              = true
      TSC: time stamp counter                = true
      RDMSR and WRMSR support                = true
      PAE: physical address extensions       = false
      MCE: machine check exception           = true
      CMPXCHG8B inst.                        = true
      APIC on chip                           = false
      SYSENTER and SYSEXIT                   = true
      MTRR: memory type range registers      = true
      PTE global bit                         = true
      MCA: machine check architecture        = true
      CMOV: conditional move/compare instr   = true
      PAT: page attribute table              = true
      PSE-36: page size extension            = false
      PSN: processor serial number           = false
      CLFLUSH instruction                    = true
      DS: debug store                        = true
      ACPI: thermal monitor and clock ctrl   = true
      MMX Technology                         = true
      FXSAVE/FXRSTOR                         = true
      SSE extensions                         = true
      SSE2 extensions                        = true
      SS: self snoop                         = false
      hyper-threading / multi-core supported = false
      TM: therm. monitor                     = true
      IA64                                   = false
      PBE: pending break event               = true
   feature information (1/ecx):
      PNI/SSE3: Prescott New Instructions     = false
      PCLMULDQ instruction                    = false
      DTES64: 64-bit debug store              = false
      MONITOR/MWAIT                           = false
      CPL-qualified debug store               = false
      VMX: virtual machine extensions         = false
      SMX: safer mode extensions              = false
      Enhanced Intel SpeedStep Technology     = true
      TM2: thermal monitor 2                  = true
      SSSE3 extensions                        = false
      context ID: adaptive or shared L1 data  = false
      SDBG: IA32_DEBUG_INTERFACE              = false
      FMA instruction                         = false
      CMPXCHG16B instruction                  = false
      xTPR disable                            = false
      PDCM: perfmon and debug                 = false
      PCID: process context identifiers       = false
      DCA: direct cache access                = false
      SSE4.1 extensions                       = false
      SSE4.2 extensions                       = false
      x2APIC: extended xAPIC support          = false
      MOVBE instruction                       = false
      POPCNT instruction                      = false
      time stamp counter deadline             = false
      AES instruction                         = false
      XSAVE/XSTOR states                      = false
      OS-enabled XSAVE/XSTOR                  = false
      AVX: advanced vector extensions         = false
      F16C half-precision convert instruction = false
      RDRAND instruction                      = false
      hypervisor guest status                 = false
   cache and TLB information (2):
      0xb0: instruction TLB: 4K, 4-way, 128 entries
      0xb3: data TLB: 4K pages, 4-way, 128 entries
      0x02: instruction TLB: 4M pages, 4-way, 2 entries
      0x87: L2 cache: 1M, 8-way, 64 byte lines
      0x30: L1 cache: 32K, 8-way, 64 byte lines
      0x04: data TLB: 4M pages, 4-way, 8 entries
      0x2c: L1 data cache: 32K, 8-way, 64 byte lines
   extended feature flags (0x80000001/edx):
      SYSCALL and SYSRET instructions        = false
      execution disable                      = false
      1-GB large page support                = false
      RDTSCP                                 = false
      64-bit extensions technology available = false
   Intel feature flags (0x80000001/ecx):
      LAHF/SAHF supported in 64-bit mode     = false
      LZCNT advanced bit manipulation        = false
      3DNow! PREFETCH/PREFETCHW instructions = false
   brand = "        Intel(R) Pentium(R) M processor 1600MHz"
   (instruction supported synth):
      MWAIT = false
   (multi-processing synth) = none
   (multi-processing method) = Intel leaf 1
   (uarch synth) = Intel {P6 Pentium M}, .13um
   (synth) = Intel Pentium M (Banias B1) {P6 Pentium M}, .13um

kcpuid (in kernel tree)

kcpuid is a small utility shipped with the Linux kernel tree in directory tools/arch/x86/kcpuid (gitweb). It appears installable with make install, though one can run it in-place:

$ cd tools/arch/x86/kcpuid
$ make
cc -Wextra -O2 -Wall -I../../../include -o kcpuid kcpuid.c
$./kcpuid -d
CPU features:

         EBX: 0x756e6547
         ECX: 0x6c65746e
         EDX: 0x49656e69

         eist                - Enhanced Intel SpeedStep Technology
         tm2                 - Thermal Monitor 2
         fpu                 - x87 FPU on chip
         vme                 - Virtual-8086 Mode Enhancement
         de                  - Debugging Extensions
         pse                 - Page Size Extensions
         tsc                 - Time Stamp Counter
         msr                 - RDMSR and WRMSR Support
         mce                 - Machine Check Exception
         cx8                 - CMPXCHG8B instr
         sep                 - SYSENTER and SYSEXIT instrs
         mtrr                - Memory Type Range Registers
         pge                 - Page Global Bit
         mca                 - Machine Check Architecture
         cmov                - Conditional Move Instrs
         pat                 - Page Attribute Table
         clflush             - CLFLUSH instr
         ds                  - Debug Store
         acpi                - Thermal Monitor and Software Controlled Clock Facilities
         mmx                 - Intel MMX Technology
         fxsr                - XSAVE and FXRSTOR Instrs
         sse                 - SSE
         sse2                - SSE2
         tm                  - Thermal Monitor
         pbe                 - Pending Break Enable

         EAX: 0x02b3b001
         EBX: 0x00000000
         ECX: 0x00000000
         EDX: 0x2c043087

         EAX: 0x80000004
         EBX: 0x00000000
         ECX: 0x00000000
         EDX: 0x00000000

         EAX: 0x20202020
         EBX: 0x20202020
         ECX: 0x65746e49
         EDX: 0x2952286c

         EAX: 0x6e655020
         EBX: 0x6d756974
         ECX: 0x20295228
         EDX: 0x7270204d

         EAX: 0x7365636f
         EBX: 0x20726f73
         ECX: 0x30303631
         EDX: 0x007a484d


CPUID is an interesting x86 instruction for it does nothing, but tell a tale of what the gear can do, if given the right software. Published on a 20 year old computer using a modern publishing workflow, thanks to the right software and CPUID detected CPU features.

What’s worse than a fire on a boat? A fire aboard an air balloon. Rip my app.

Affected Apps:

  • sillypaste-db

A server hosting some of your apps has suffered irreparable hardware damage. Please migrate your Fly Machines to other hosts and restore volumes from any backups.

All good things come to an end, including this pastebin project. If I find myself using it again, I may spin up a fresh database if that opportunity presents itself.

I wouldn’t recommend at this time. Having been erroneously billed (then quickly refunded) a couple times, fought with their V1 stack and data corruption issues, and it goes on. At one time I ended up with the PostgresSQL database container crashing with Out of Memory… despite issuing countless scale commands. Eventually I paid for the $20/mo email support for the support staff to tell me flatly - you must migrate and yes v1 is still considered robust for some workloads. I’m pretty sure the reliability claim is a white lie, but indeed, I had to upgrade. Turned out to be tricky due to inconsistent documentation on V1 vs V2 features. Had to ask another question.

It should be noted that support was quick to reply to my email because I paid for the $20 email support plan; support on the community forums is best effort and will not lead to a fruitful support conversation.

What went well with

Now that’s critical negative review has been written, let’s review some of’s wins.

  • flyctl command line tool compares favorable against heroku. Simpler operation, less features to confound over.
  • Its web design is modern and nice… but like many UIs designed for touch input, it suffers from low density… which pushed me to use the CLI wherever possible.
  • Support forum uses Discourse, a wonderful no-nonsense support forum experience.
  • When it works, it works really well.
  • It was a cheap experiment. You do get what you pay for (free hosting = it can burn down at any time and that’s ok).

Alternatives for hosting Sillypaste

I have no first-hand endorsements, but next I’d assess AWS Lambda, Digital Ocean Apps, or a simple VPS for hosting this modest Django webapp. A service that gets billing right, maintains consistent documentation, and has a working reliable PaaS offering is ideal.

For now, Sillypaste shall remain offline but the code is always available on GitHub. All I need to do to remove the DNS entry is delete it from my terraform project then run terraform apply.

Figure 1: John from USA - CC-BY-2.0

Figure 1: John from USA - CC-BY-2.0

Watch out, things break, stuff catches fire. Let’s talk about backups.

Last post, I stated that I’m going to switch focus away from NixOS commentary. This is still the plan. Today, I am still committed to NixOS thanks to technical debt created - migrations aren’t for free. Until then, enjoy my NixOS posting :).

Last fall, I wanted to reformat my laptop’s NixOS deployment from BTRFS (encased within LVM2 itself encased in LUKS) to a ZFS partition plus another swap partition. My Nix install is comprised of a few artifacts:

  1. My git repository with the flake.nix and flake.lock files
  2. The workstation’s /secrets folder, sensitive data for service accounts.
  3. The workstation’s /home folder

Both /secrets and /home are backed up via borgmatic (using borg) on a nightly basis via a crufty old nixos module that I wrote (example of usage). Both folders were also snapshotted by BTRBK every 15 minutes (via this nixos configuration). This frequent snapshotting policy will continue on the ZFS reinstall powered by zfs-autosnapshot.

The first test was to verify the integrity of the backed up artifacts. I was able to execute a full restore from backup from within a virtual machine. This included adapting my laptop’s flake configuration to the VM, rebuilding, then executing the borg extract commands.

Fun fact: borg mount and rsync is several times slower than running borg extract (using BorgBase). Keep that in mind when executing restores - if you need a full restore or a restore of a subdirectory, consider borg extract. If you need to pick and choose many files, consider borg mount.

After the successful test restore, it was time to execute a final backup. On my setup that’s as simple as systemctl start backup. Then boot a NixOS installer. Invoking parted /dev/nvme0n1, I came up with the following partition layout:

Model: INTEL SSDPEKNU010TZ (nvme)
Disk /dev/nvme0n1: 1024GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  1000MB  999MB   fat32        boot  boot, esp
 2      1000MB  1007GB  1006GB               zfs
 3      1007GB  1023GB  16.2GB               swap

The swap partition is used in conjunction with NixOS’s swapDevices.*.randomEncryption.enable setting. This swap partition is encrypted using LUKS. This encrypted device mapper device is used for swap.

Then I followed my standard install instructions here on this blog.

Recovery strategy

As part of this restore procedure, I tested my restore strategy. It turned out the thumb drive and QR code with the full disk encryption (FDE) key for the thumb drive were compromised. They simply didn’t work - the QR code was of a different key.

Had I needed to recover my setup in a data loss scenario, it is likely I would had lost data due to not having access to recovery material. I was at risk of data loss. Ooop!

Figure 2: Alex E. Proimos - CC-BY-2.0

Figure 2: Alex E. Proimos - CC-BY-2.0

Next I created new recovery material. It consisted of two components: A passphrase and an encrypted thumb drive. They live together; the passphrase is more of a “are you sure you want to open this?” than a security measure. The encrypted thumb drive contains my PGP private keys (encrypted with each key’s own passphrase) and password database encrypted against my “private use only” key.

In order to restore from this media, first open the LUKS container via cryptsetup luksOpen /dev/disk/by-id/usb-...-part0 usbCrypt then mount it via mount /dev/mapper/usbCrypt /mnt/usb. I can load the GPG keys into my gpg then run gpg --decrypt --output - /mnt/usb/password-store/backups/stargate/passphrase to get the backup borg storage passphrase. I can then set up borg to access my backup via accessing my backup service’s dashboard.

Finally I was able to run borg extract ... to kick off the restore on the laptop.

From start to finish the redeploy and restore took 3 hours for the data restore and another hour due to various tasks of how this procedure works. It’s not super automatic, but hey, it’s tested and it works!

Well I guess the restore worked!

Figure 3: Data loss is possible with a sledge hammer and no backups

Figure 3: Data loss is possible with a sledge hammer and no backups

Test your backups. Until you do, they are but a speculative investment; you’re not sure if they work. In theory they say they should, however, who really knows. Nobody really knows. Go test your backups. Haven’t done it yet, well, then, buy this sledge hammer and apply it to your storage devices, because your data is worthless without tested backups - it could disappear at any time. Theft, fire, PEBKAC, Software Bug (like Steam’s infamous rm -rf script bug)… anything is possible.

Want a T-shirt?

I’m selling T-shirts with that sledge hammer fellow on the back and “COMPUTERS WERE A MISTAKE” on the front. Of course it’s supposed to be cheeky and not too serious - we must laugh at technology before it destroys our human identity. And embrace the good parts. Computers are fun, if you let ’em. Have fun with ’em, but don’t let ’em control every aspect of your being.