On Thu, Jul 4, 2024 at 2:26 AM Michael Roth <michael.r...@amd.com> wrote: > > Michael, any ideas? Is there a way for the host to retrieve the supported > > CPUID bits for SEV-SNP guests? > > If we want to support -cpu host, then I don't really see a way around > needing to maintain a filter of some sort sanitize what gets passed to > firmware. Generally, every new CPU model is likely to have some features > which might be a liability security-wise to allow in SNP guests, so the > CPUID validation is sort of a whitelist of curated features that make > sense for guests and can be enabled securely in the context of SNP. > > Everything else would need to be filtered out, so we'd need to keep that > list constantly updated.
It would be per new model and right now there are only a handful of bits that have to be blocked; so it wouldn't be particularly bad. > I think that may be possible, but do we have a strong use-case for > supporting -cpu host in conjunction with SNP guests that this would be > a worthwhile endeavor? It's a common way to launch a guest if you're not interested in migration (which is obviously the case for SNP right now), so it's more like "why not". :) > > One possibility is to set up a fake guest---either in QEMU or when KVM > > starts---to do a LAUNCH_UPDATE for the CPUID page, but even that is not > > perfect. For example, I got > > Yah, the firmware-provided responses are more of a debug tool and not > something I think we can rely on to enumerate capabilities. > > You could in theory take the ruleset in the PPR (Chapter 2, CPUID Policy > Enforcement), turn that into something programmatic, and apply that > against the host's CPUID values, but the policies are a bit more > specific in some cases, and the PPR is per-CPU-model so both the rules > and inputs can change from one host to the next. Yeah, and if you mix that with knowledge of what KVM can/cannot virtualize that doesn't exist in the processor (which isn't that much), then you end up with something a lot like patch 2 It would be nice if the policy enforcement were changed to allow the TSC deadline timer and X2APIC bits (you probably don't want TSC adjust, that's the right call; and virt SSBD is not accessible because you use V_SPEC_CTRL instead). But then there would be no way to find out if the change actually happened. > So I don't see a great way to leverage that to make things easier here. > The manually-maintained filter you've proposed here seems more reliable > to me. Yep, I think I'll include that patch as the maintainability doesn't seem bad. Paolo
