CPU Type

[laxarus]

I am not too sure about what type of CPU to select considering my case.
I need live migration and one proxmox host has the 2 x Xeon Scalable 5th gen while the other one has a Raptor Lake.
Which type of VM CPU architecture would be best? ChatGPT recommends x86-64-v3 but I am not too sure?

 

[SteveITS]

I found the Proxmox list has at least one error. There is a list at https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html which should help also.

 

[fiona]

Hi,

I found the Proxmox list has at least one error.

could you tell us what the error is?

 

[SteveITS]

Sure.... previously posted in https://forum.proxmox.com/threads/c...-slower-than-x86-64-v2-aes.159107/post-766479.

The wiki shows "x86-64-v3: Compatible with Intel CPU >= Broadwell" but the QEMU list shows all four (earlier) Haswell versions are compatible. In testing, v3 works just fine, live migrating back and forth.

Haswell-noTSX works on this Xeon E5-2650 v3 CPU also.

Interestingly, plain "Haswell" fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.hle [bit 4]
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.rtm [bit 11]

Haswell-noTSX-IBRS fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EDX.spec-ctrl [bit 26]

Haswell-IBRS fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.hle [bit 4]
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.rtm [bit 11]
kvm: warning: host doesn't support requested feature: CPUID.07H:EDX.spec-ctrl [bit 26]

 

[fiona]

Sure.... previously posted in https://forum.proxmox.com/threads/c...-slower-than-x86-64-v2-aes.159107/post-766479.

The wiki shows "x86-64-v3: Compatible with Intel CPU >= Broadwell" but the QEMU list shows all four (earlier) Haswell versions are compatible. In testing, v3 works just fine, live migrating back and forth.

Haswell-noTSX works on this Xeon E5-2650 v3 CPU also.

Thanks! A patch was proposed: https://lore.proxmox.com/pve-devel/20250513080108.17210-1-f.ebner@proxmox.com/

Interestingly, plain "Haswell" fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.hle [bit 4]
kvm: warning: host doesn't support requested feature: CPUID.07H:EBX.rtm [bit 11]

Those are exaclty the features turned off by -noTSX.

Haswell-noTSX-IBRS fails with:
kvm: warning: host doesn't support requested feature: CPUID.07H:EDX.spec-ctrl [bit 26]

And your CPU doesn't support the additional feature required for IBRS.

But none of those features are relevant for x86-64-v3 compatibility.

 

[guruevi]

@SteveITS: all those feature bits are issues in speculative control they are nowadays disabled either at firmware level, kernel level or both. So you cannot access those processor bits unless you turn speculative control protections off (not recommended). The -noTSX flavor is the safest to use, but theoretically you could use the other ones with the right host kernel settings.