Re: Versioned CPU types in libvirt
On 10/28/23 10:49 AM, Jonathon Jongsma wrote:
I'm currently looking at getting libvirt working with AMD's SEV-SNP
encrypted virtualization technology. I have access to a test machine
with an AMD EPYC 7713 processor which I can use to launch SNP guests
with qemu, but only when I specify one of the following versioned -cpu
values:
- EPYC-v4
- EPYC-Milan-v2
- EPYC-Rome-v3
From what I understand, the unversioned CPU models in qemu are supposed
to resolve to a specific versioned CPU model depending on the machine
type. But I'm not exactly sure how machine type influences it.
At qemu level that's what I thought too, and it sounds like that was the
eventual plan, but it's not implemented yet. `qemu -cpu FOO` always maps
to `-cpu FOO-v1`. There's some info in qemu docs here, last sentence
explicitly makes this clear it may change in future
https://www.qemu.org/docs/master/about/deprecated.html#runnability-guaran...
Milan-v2 does add some new CPU features, but the one SNP related bit
those models change is `complex_indexing` in l3_cache cpuid, whatever
that is. Doesn't look overrideable on qemu command line, or anything
libvirt detects from host.
I've got some libvirt patches to launch an SEV-SNP guest working
now
except for the CPU model specification. As far as I can tell, I can
currently only specify the un-versioned model in libvirt. Is there any
way to request a particular versioned CPU from qemu? I feel like I'm
missing something here.
The reason this fails is that SNP hardware explicitly rejects some guest
CPUID values that it deems unsafe. That `complex_indexing` bit is one of
them, but `-cpu host` triggers more. TDX seems to have a similar
mechanism, but it looks like qemu code is filtering those out for -cpu
host. Maybe qemu SNP can do the same, I'll ask AMD devs
But yes in the meantime a libvirt workaround would help
Thanks,
Cole
I should perhaps also mention that I'm running a development
version of
qemu from Cole's copr repo[1], which could still have some related bugs
[1]
https://copr.fedorainfracloud.org/coprs/g/virtmaint-sig/sev-snp-coconut/
Thanks,
Jonathon