Question about cpu-models and cpu flags

udo · 2026-08-17T16:29:27+0200

Hi,
we have cluster with mixed CPUs and we use as cpu-settings something like this (amd)

Code:

cpu: x86-64-v3,flags=+ibpb;+virt-ssbd;+amd-ssbd

to avoid to select the additional flags I tried to define an custom cpu type.
Two things are not working:
1. the cpu type x86-64-v3 isn't supported (unable to parse value of 'reported-model': value 'x86-64-v3' does not have a value in the enumeration)
2. the flag lahf_lm was not added. In /proc/cpuinfo it's named lahf_lm - the flag in the gui is named lahf-lm, but both entrys don't show the flag inside the VM.

Code:

cat /etc/pve/virtual-guest/cpu-models.conf
cpu-model: amd-v3
    flags +ssse3;+fma;+sse4_1;+sse4_2;+movbe;+popcnt;+aes;+xsave;+avx;+f16c;+lahf_lm;+abm;+bmi1;+avx2;+bmi2;-vme;+amd-ssbd;+ibpb;+virt-ssbd
    hidden 0
    reported-model x86-64-v3

And with the x86-64-v3 I see inside the VM the additional entry amd_e400 in the bugs list, which is missing in my custom-cpu.

Best regards

Udo

abied-charreton · 2026-08-17T17:07:35+0200

Hello Udo,

udo said:
1. the cpu type x86-64-v3 isn't supported (unable to parse value of 'reported-model': value 'x86-64-v3' does not have a value in the enumeration)

That is expected, x86-64-v3 is not an existing base model as far as QEMU is concerned. It is a built-in predefined model we expand when generating the QEMU command line, so it cannot be used as reported-model for a custom model.

udo said:
2. the flag lahf_lm was not added. In /proc/cpuinfo it's named lahf_lm - the flag in the gui is named lahf-lm, but both entrys don't show the flag inside the VM.

Yes, we normalize the flags to their QEMU form, which does not always match /proc/cpuinfo, see [0]. In a quick test here, setting the lahf-lm flag on a custom model does show up in the guest as lahf_lm in /proc/cpuinfo.

udo said:
And with the x86-64-v3 I see inside the VM the additional entry amd_e400 in the bugs list, which is missing in my custom-cpu.

bugs is not a flag list, as far as I know that is derived by the kernel from the CPU model and the flags that are visible. It is not set explicitly.

[0] https://lore.proxmox.com/all/20260515092839.238064-6-a.bied-charreton@proxmox.com/

udo · 2026-08-17T18:10:49+0200

Hi Arthur,
thanks for the info.
The lahf-lm flag is strange. Have testet a limitied cpu with "flags +aes;+lahf-lm" on an Intel-Node (E5-2640 v4) and the flag is visible inside the VM. Then I migrate the VM back to an AMD-Node (EPYC 9454P) and start the VM there… and the lahf-lm flag is gone! The host show the flag in cpu-info.
Both Nodes (intel + AMD) are fresh updated - pve-manager/9.2.4/5e5ae681198514d4 (running kernel: 7.0.14-11-pve).
The same on another host with an EPYC 9455.

abied-charreton · 2026-08-18T08:54:39+0200

Hello Udo,

that is interesting. Could you please share the output of

Code:

qemu-system-x86_64 -cpu help
pvesh get /cluster/qemu/cpu-flags --output-format json-pretty

on the AMD node?

udo · 2026-08-18T12:12:35+0200

Hello Arthur,
of course…
lahf-am is there:

Code:

qemu-system-x86_64 -cpu help
Available CPUs:
  486                   (alias configured by machine type)
  486-v1                
  Broadwell             (alias configured by machine type)
  Broadwell-IBRS        (alias of Broadwell-v3)
  Broadwell-noTSX       (alias of Broadwell-v2)
  Broadwell-noTSX-IBRS  (alias of Broadwell-v4)
  Broadwell-v1          Intel Core Processor (Broadwell)
  Broadwell-v2          Intel Core Processor (Broadwell, no TSX)
  Broadwell-v3          Intel Core Processor (Broadwell, IBRS)
  Broadwell-v4          Intel Core Processor (Broadwell, no TSX, IBRS)
  Cascadelake-Server    (alias configured by machine type)
  Cascadelake-Server-noTSX  (alias of Cascadelake-Server-v3)
  Cascadelake-Server-v1  Intel Xeon Processor (Cascadelake)
  Cascadelake-Server-v2  Intel Xeon Processor (Cascadelake) [ARCH_CAPABILITIES]
  Cascadelake-Server-v3  Intel Xeon Processor (Cascadelake) [ARCH_CAPABILITIES, no TSX]
  Cascadelake-Server-v4  Intel Xeon Processor (Cascadelake) [ARCH_CAPABILITIES, EPT switching, no TSX]
  Cascadelake-Server-v5  Intel Xeon Processor (Cascadelake) [ARCH_CAPABILITIES, EPT switching, XSAVES, no TSX]
  ClearwaterForest      (alias configured by machine type)
  ClearwaterForest-v1   Intel Xeon Processor (ClearwaterForest)
  ClearwaterForest-v2   Intel Xeon Processor (ClearwaterForest) [with cet-ss and cet-ibt]
  ClearwaterForest-v3   Intel Xeon Processor (ClearwaterForest) [with its-no, cwf-ap cache model and 0x1f leaf]
  Conroe                (alias configured by machine type)
  Conroe-v1             Intel Celeron_4x0 (Conroe/Merom Class Core 2)
  Cooperlake            (alias configured by machine type)
  Cooperlake-v1         Intel Xeon Processor (Cooperlake)
  Cooperlake-v2         Intel Xeon Processor (Cooperlake) [XSAVES]
  Denverton             (alias configured by machine type)
  Denverton-v1          Intel Atom Processor (Denverton)
  Denverton-v2          Intel Atom Processor (Denverton) [no MPX, no MONITOR]
  Denverton-v3          Intel Atom Processor (Denverton) [XSAVES, no MPX, no MONITOR]
  Dhyana                (alias configured by machine type)
  Dhyana-v1             Hygon Dhyana Processor
  Dhyana-v2             Hygon Dhyana Processor [XSAVES]
  DiamondRapids         (alias configured by machine type)
  DiamondRapids-v1      Intel Xeon Processor (DiamondRapids)
  EPYC                  (alias configured by machine type)
  EPYC-Genoa            (alias configured by machine type)
  EPYC-Genoa-v1         AMD EPYC-Genoa Processor
  EPYC-Genoa-v2         AMD EPYC-Genoa-v2 Processor
  EPYC-IBPB             (alias of EPYC-v2)
  EPYC-Milan            (alias configured by machine type)
  EPYC-Milan-v1         AMD EPYC-Milan Processor
  EPYC-Milan-v2         AMD EPYC-Milan-v2 Processor
  EPYC-Milan-v3         AMD EPYC-Milan-v3 Processor
  EPYC-Rome             (alias configured by machine type)
  EPYC-Rome-v1          AMD EPYC-Rome Processor
  EPYC-Rome-v2          AMD EPYC-Rome Processor
  EPYC-Rome-v3          AMD EPYC-Rome-v3 Processor
  EPYC-Rome-v4          AMD EPYC-Rome-v4 Processor (no XSAVES)
  EPYC-Rome-v5          AMD EPYC-Rome-v5 Processor
  EPYC-Turin            (alias configured by machine type)
  EPYC-Turin-v1         AMD EPYC-Turin Processor
  EPYC-v1               AMD EPYC Processor
  EPYC-v2               AMD EPYC Processor (with IBPB)
  EPYC-v3               AMD EPYC Processor
  EPYC-v4               AMD EPYC-v4 Processor
  EPYC-v5               AMD EPYC-v5 Processor
  GraniteRapids         (alias configured by machine type)
  GraniteRapids-v1      Intel Xeon Processor (GraniteRapids)
  GraniteRapids-v2      Intel Xeon Processor (GraniteRapids)
  GraniteRapids-v3      Intel Xeon Processor (GraniteRapids) [with gnr-sp cache model and 0x1f leaf]
  GraniteRapids-v4      Intel Xeon Processor (GraniteRapids) [with cet-ss and cet-ibt]
  GraniteRapids-v5      Intel Xeon Processor (GraniteRapids) [with its-no]
  Haswell               (alias configured by machine type)
  Haswell-IBRS          (alias of Haswell-v3)
  Haswell-noTSX         (alias of Haswell-v2)
  Haswell-noTSX-IBRS    (alias of Haswell-v4)
  Haswell-v1            Intel Core Processor (Haswell)
  Haswell-v2            Intel Core Processor (Haswell, no TSX)
  Haswell-v3            Intel Core Processor (Haswell, IBRS)
  Haswell-v4            Intel Core Processor (Haswell, no TSX, IBRS)
  Icelake-Server        (alias configured by machine type)
  Icelake-Server-noTSX  (alias of Icelake-Server-v2)
  Icelake-Server-v1     Intel Xeon Processor (Icelake)
  Icelake-Server-v2     Intel Xeon Processor (Icelake) [no TSX]
  Icelake-Server-v3     Intel Xeon Processor (Icelake)
  Icelake-Server-v4     Intel Xeon Processor (Icelake)
  Icelake-Server-v5     Intel Xeon Processor (Icelake) [XSAVES]
  Icelake-Server-v6     Intel Xeon Processor (Icelake) [5-level EPT]
  Icelake-Server-v7     Intel Xeon Processor (Icelake) [TSX, taa-no]
  IvyBridge             (alias configured by machine type)
  IvyBridge-IBRS        (alias of IvyBridge-v2)
  IvyBridge-v1          Intel Xeon E3-12xx v2 (Ivy Bridge)
  IvyBridge-v2          Intel Xeon E3-12xx v2 (Ivy Bridge, IBRS)
  KnightsMill           (alias configured by machine type)
  KnightsMill-v1        Intel Xeon Phi Processor (Knights Mill)
  Nehalem               (alias configured by machine type)
  Nehalem-IBRS          (alias of Nehalem-v2)
  Nehalem-v1            Intel Core i7 9xx (Nehalem Class Core i7)
  Nehalem-v2            Intel Core i7 9xx (Nehalem Core i7, IBRS update)
  Opteron_G1            (alias configured by machine type)
  Opteron_G1-v1         AMD Opteron 240 (Gen 1 Class Opteron)
  Opteron_G2            (alias configured by machine type)
  Opteron_G2-v1         AMD Opteron 22xx (Gen 2 Class Opteron)
  Opteron_G3            (alias configured by machine type)
  Opteron_G3-v1         AMD Opteron 23xx (Gen 3 Class Opteron)
  Opteron_G4            (alias configured by machine type)
  Opteron_G4-v1         AMD Opteron 62xx class CPU
  Opteron_G5            (alias configured by machine type)
  Opteron_G5-v1         AMD Opteron 63xx class CPU
  Penryn                (alias configured by machine type)
  Penryn-v1             Intel Core 2 Duo P9xxx (Penryn Class Core 2)
  SandyBridge           (alias configured by machine type)
  SandyBridge-IBRS      (alias of SandyBridge-v2)
  SandyBridge-v1        Intel Xeon E312xx (Sandy Bridge)
  SandyBridge-v2        Intel Xeon E312xx (Sandy Bridge, IBRS update)
  SapphireRapids        (alias configured by machine type)
  SapphireRapids-v1     Intel Xeon Processor (SapphireRapids)
  SapphireRapids-v2     Intel Xeon Processor (SapphireRapids)
  SapphireRapids-v3     Intel Xeon Processor (SapphireRapids)
  SapphireRapids-v4     Intel Xeon Processor (SapphireRapids) [with spr-sp cache model and 0x1f leaf]
  SapphireRapids-v5     Intel Xeon Processor (SapphireRapids) [with cet-ss and cet-ibt]
  SapphireRapids-v6     Intel Xeon Processor (SapphireRapids) [with its-no]
  SierraForest          (alias configured by machine type)
  SierraForest-v1       Intel Xeon Processor (SierraForest)
  SierraForest-v2       Intel Xeon Processor (SierraForest)
  SierraForest-v3       Intel Xeon Processor (SierraForest) [with srf-sp cache model and 0x1f leaf]
  SierraForest-v4       Intel Xeon Processor (SierraForest) [with cet-ss and cet-ibt]
  SierraForest-v5       Intel Xeon Processor (SierraForest) [with its-no]
  Skylake-Client        (alias configured by machine type)
  Skylake-Client-IBRS   (alias of Skylake-Client-v2)
  Skylake-Client-noTSX-IBRS  (alias of Skylake-Client-v3)
  Skylake-Client-v1     Intel Core Processor (Skylake)
  Skylake-Client-v2     Intel Core Processor (Skylake, IBRS)
  Skylake-Client-v3     Intel Core Processor (Skylake, IBRS, no TSX)
  Skylake-Client-v4     Intel Core Processor (Skylake, IBRS, no TSX) [IBRS, XSAVES, no TSX]
  Skylake-Server        (alias configured by machine type)
  Skylake-Server-IBRS   (alias of Skylake-Server-v2)
  Skylake-Server-noTSX-IBRS  (alias of Skylake-Server-v3)
  Skylake-Server-v1     Intel Xeon Processor (Skylake)
  Skylake-Server-v2     Intel Xeon Processor (Skylake, IBRS)
  Skylake-Server-v3     Intel Xeon Processor (Skylake, IBRS, no TSX)
  Skylake-Server-v4     Intel Xeon Processor (Skylake, IBRS, no TSX) [IBRS, EPT switching, no TSX]
  Skylake-Server-v5     Intel Xeon Processor (Skylake, IBRS, no TSX) [IBRS, XSAVES, EPT switching, no TSX]
  Snowridge             (alias configured by machine type)
  Snowridge-v1          Intel Atom Processor (SnowRidge)
  Snowridge-v2          Intel Atom Processor (Snowridge, no MPX)
  Snowridge-v3          Intel Atom Processor (Snowridge, no MPX) [XSAVES, no MPX]
  Snowridge-v4          Intel Atom Processor (Snowridge, no MPX) [no split lock detect, no core-capability]
  Westmere              (alias configured by machine type)
  Westmere-IBRS         (alias of Westmere-v2)
  Westmere-v1           Westmere E56xx/L56xx/X56xx (Nehalem-C)
  Westmere-v2           Westmere E56xx/L56xx/X56xx (IBRS update)
  YongFeng              (alias configured by machine type)
  YongFeng-v1           Zhaoxin YongFeng Processor
  YongFeng-v2           Zhaoxin YongFeng Processor [with the correct model number]
  YongFeng-v3           Zhaoxin YongFeng Processor [with the cache model and 0x1f leaf]
  athlon                (alias configured by machine type)
  athlon-v1             QEMU Virtual CPU version 2.5+
  core2duo              (alias configured by machine type)
  core2duo-v1           Intel(R) Core(TM)2 Duo CPU     T7700  @ 2.40GHz
  coreduo               (alias configured by machine type)
  coreduo-v1            Genuine Intel(R) CPU           T2600  @ 2.16GHz
  kvm32                 (alias configured by machine type)
  kvm32-v1              Common 32-bit KVM processor
  kvm64                 (alias configured by machine type)
  kvm64-v1              Common KVM processor
  n270                  (alias configured by machine type)
  n270-v1               Intel(R) Atom(TM) CPU N270   @ 1.60GHz
  pentium               (alias configured by machine type)
  pentium-v1            
  pentium2              (alias configured by machine type)
  pentium2-v1          
  pentium3              (alias configured by machine type)
  pentium3-v1          
  phenom                (alias configured by machine type)
  phenom-v1             AMD Phenom(tm) 9550 Quad-Core Processor
  qemu32                (alias configured by machine type)
  qemu32-v1             QEMU Virtual CPU version 2.5+
  qemu64                (alias configured by machine type)
  qemu64-v1             QEMU Virtual CPU version 2.5+
  base                  base CPU model type with no features enabled
  host                  processor with all supported host features
  max                   Enables all features supported by the accelerator in the current host

Recognized CPUID flags:
  3dnow 3dnowext 3dnowprefetch abm ace2 ace2-en acpi adx aes amd-no-ssb
  amd-psfd amd-ssbd amd-stibp amx-avx512 amx-bf16 amx-bf16-alias
  amx-complex amx-complex-alias amx-fp16 amx-fp16-alias amx-fp8 amx-int8
  amx-int8-alias amx-movrs amx-tf32 amx-tile apic apx-nci-ndd-nf apxf arat
  arch-capabilities arch-lbr auto-ibrs avic avx avx-ifma avx-ne-convert
  avx-vnni avx-vnni-int16 avx-vnni-int8 avx10 avx10-128 avx10-256 avx10-512
  avx10-vnni-int avx2 avx512-4fmaps avx512-4vnniw avx512-bf16 avx512-fp16
  avx512-vp2intersect avx512-vpopcntdq avx512bitalg avx512bw avx512cd
  avx512dq avx512er avx512f avx512ifma avx512pf avx512vbmi avx512vbmi2
  avx512vl avx512vnni bhi-ctrl bhi-no bmi1 bmi2 bus-lock-detect cet-ibt
  cet-ss cid cldemote clflush clflushopt clwb clzero cmov cmp-legacy
  cmpccxadd core-capability cr8legacy cx16 cx8 dca ddpd-u de decodeassists
  ds ds-cpl dtes64 eraps erms est extapic f16c fb-clear fbsdp-no
  fdp-excptn-only flush-l1d flushbyasid fma fma4 fpu fred fs-gs-base-ns
  fsgsbase fsrc fsrm fsrs full-width-write fxsr fxsr-opt fzrm gds-no gfni
  gmet hle ht hypervisor ia64 ibpb ibpb-brtype ibrs ibrs-all ibs intel-psfd
  intel-pt intel-pt-lip invpcid invtsc ipred-ctrl its-no kvm-asyncpf
  kvm-asyncpf-int kvm-asyncpf-vmexit kvm-hint-dedicated kvm-mmu
  kvm-msi-ext-dest-id kvm-nopiodelay kvm-poll-control kvm-pv-eoi kvm-pv-ipi
  kvm-pv-sched-yield kvm-pv-tlb-flush kvm-pv-unhalt kvm-steal-time kvmclock
  kvmclock kvmclock-stable-bit la57 lahf-lm lam lbrv
  lfence-always-serializing lkgs lm lwp mca mcdt-no mce md-clear mds-no
  misalignsse mmx mmxext monitor movbe movdir64b movdiri movrs mpx msr
  msr-imm mtrr no-nested-data-bp nodeid-msr npt nrip-save null-sel-clr-base
  nx osvw overflow-recov pae pat pause-filter pbe pbrsb-no pcid pclmulqdq
  pcommit pdcm pdpe1gb perfctr-core perfctr-nb perfmon-v2 pfthreshold pge
  phe phe-en pks pku pmm pmm-en pn pni popcnt prefetchi prefetchiti
  pschange-mc-no psdp-no pse pse36 rdctl-no rdpid rdrand rdseed rdtscp
  rfds-clear rfds-no rrsba-ctrl rsba rtm sbdr-ssdp-no sbpb sep serialize
  sgx sgx-aex-notify sgx-debug sgx-edeccssa sgx-exinfo sgx-kss sgx-mode64
  sgx-provisionkey sgx-tokenkey sgx1 sgx2 sgxlc sha-ni sha512 skinit
  skip-l1dfl-vmentry sm3 sm4 smap smep smx spec-ctrl split-lock-detect
  srso-no srso-user-kernel-no ss ssb-no ssbd sse sse2 sse4.1 sse4.2 sse4a
  ssse3 stibp stibp-always-on succor svm svm-lock svme-addr-chk syscall
  taa-no tbm tce tm tm2 topoext tsa-l1-no tsa-sq-no tsc tsc-adjust
  tsc-deadline tsc-scale tsx-ctrl tsx-ldtrk umip v-vmsave-vmload vaes
  verw-clear vgif virt-ssbd vmcb-clean vme vmx vmx-activity-hlt
  vmx-activity-shutdown vmx-activity-wait-sipi vmx-any-errcode
  vmx-apicv-register vmx-apicv-vid vmx-apicv-x2apic vmx-apicv-xapic
  vmx-cr3-load-noexit vmx-cr3-store-noexit vmx-cr8-load-exit
  vmx-cr8-store-exit vmx-desc-exit vmx-enable-user-wait-pause
  vmx-encls-exit vmx-entry-ia32e-mode vmx-entry-load-bndcfgs
  vmx-entry-load-cet vmx-entry-load-efer vmx-entry-load-fred
  vmx-entry-load-pat vmx-entry-load-perf-global-ctrl vmx-entry-load-pkrs
  vmx-entry-load-rtit-ctl vmx-entry-noload-debugctl vmx-ept vmx-ept-1gb
  vmx-ept-2mb vmx-ept-advanced-exitinfo vmx-ept-execonly vmx-eptad
  vmx-eptp-switching vmx-exit-ack-intr vmx-exit-clear-bndcfgs
  vmx-exit-clear-rtit-ctl vmx-exit-load-efer vmx-exit-load-pat
  vmx-exit-load-perf-global-ctrl vmx-exit-load-pkrs
  vmx-exit-nosave-debugctl vmx-exit-save-cet vmx-exit-save-efer
  vmx-exit-save-pat vmx-exit-save-preemption-timer vmx-exit-secondary-ctls
  vmx-flexpriority vmx-hlt-exit vmx-ins-outs vmx-intr-exit vmx-invept
  vmx-invept-all-context vmx-invept-single-context
  vmx-invept-single-context vmx-invept-single-context-noglobals
  vmx-invlpg-exit vmx-invpcid-exit vmx-invvpid vmx-invvpid-all-context
  vmx-invvpid-single-addr vmx-io-bitmap vmx-io-exit vmx-mbec
  vmx-monitor-exit vmx-movdr-exit vmx-msr-bitmap vmx-mtf vmx-mwait-exit
  vmx-nested-exception vmx-nmi-exit vmx-page-walk-4 vmx-page-walk-5
  vmx-pause-exit vmx-ple vmx-pml vmx-posted-intr vmx-preemption-timer
  vmx-rdpmc-exit vmx-rdrand-exit vmx-rdseed-exit vmx-rdtsc-exit
  vmx-rdtscp-exit vmx-secondary-ctls vmx-shadow-vmcs vmx-store-lma
  vmx-true-ctls vmx-tsc-offset vmx-tsc-scaling vmx-unrestricted-guest
  vmx-vintr-pending vmx-vmfunc vmx-vmwrite-vmexit-fields vmx-vnmi
  vmx-vnmi-pending vmx-vpid vmx-wbinvd-exit vmx-xsaves vmx-zero-len-inject
  vnmi vpclmulqdq waitpkg wbnoinvd wdt wrmsrns x2apic xcrypt xcrypt-en xfd
  xgetbv1 xop xsave xsavec xsaveerptr xsaveopt xsaves xstore xstore-en xtpr
  zero-fcs-fds

pvesh get /cluster/qemu/cpu-flags produce an output of > 4700 lines!

I guess, this is the relavant part?! The tested amd-nodes are pve10 + pve16

Code:

pvesh get /cluster/qemu/cpu-flags --output-format json-pretty | grep -B 1 -A 19 lahf  
   {
      "name" : "lahf-lm",
      "supported-on" : [
         "pve01",
         "pve02",
         "pve03",
         "pve04",
         "pve05",
         "pve06",
         "pve07",
         "pve08",
         "pve09",
         "pve10",
         "pve11",
         "pve12",
         "pve13",
         "pve14",
         "pve15",
         "pve16"
      ]
   },

Best regards

Udo

Question about cpu-models and cpu flags

udo

Distinguished Member

abied-charreton

Proxmox Staff Member

udo

Distinguished Member

abied-charreton

Proxmox Staff Member

udo

Distinguished Member

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