[pve-devel] [PATCH-SERIES qemu/qemu-server/manager 0/1] add and set x86-64-v2 as default model for new vms

[Eneko Lacunza]

Hi,

Thanks Alexandre for this series, I think they will make admin's life 
easier and Proxmox clusters more performing by default.

El 18/5/23 a las 7:03, DERUMIER, Alexandre escribió:
>> FWIW, there was a v3:
> Yes, this was last version.
>
> The end of discussion was here:
> https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg02128.html
>
>
> I wasn"t merged, because moslty, you can't have spectre/meltdown/...
> mitigation enabled cross amd/intel, and also, generally users don't mix
> intel/amd, so it's already possible to choose the lowest inte/amd cpu
> model of the cluster.
>
>
> Personnally, I think it could really be usefull for mixed intel/amd
> cluster. (as kvm64 will not work with newer distros). It's better than
> kvm64 anymay. (but no mitigation).

I don't know what experience you had with mixed Intel/AMD clusters, but 
ours has been quite bad. Only 1 core VMs seem to migrate well, if VM is 
multicore live migration has been broken years ago. We haven't tried 
with kernel 6.2, but it fixed many issues with live migration, maybe it 
fixed this Intel/AMD migration issue too, I can test it that helps.

Not that I don't appreciate its usefulness, but please be careful how 
you advertise it ;)

>
> a better way could be to auto choose the best compatible cpu model
> across cluster for user at vm creation.
>
> with some kind of new virtual "best" cpumodel at vm creation only (then
> the correct best compatible cpu model is compute and write in vm
> configuration).
>
> We could stream host cpumodel/revision/flags with pvestatd,
> then compare flags to choose the correct model.
> (mitigations autotuning could be great too)
>
> all flags are documented in libvirt here:
> https://gitlab.com/libvirt/libvirt/-/blob/master/src/cpu_map/
>
>
> the simple algo is something like:
>   - if nocluster/single node, set cpumodel to max/host
>   - elsif cluster have only intel hosts, find best intel cpumodel
>   - elsif cluster have only amd hosts, find best amd cpumodel
>   - elsif cluster have mixed intel/amd hosts, find best x64-64-vX cpu
> model

Of course this would be best for admins... :-)

Thanks

Eneko Lacunza
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