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-guarantee... 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