Re: [PATCH 2/4] selftests/kvm: check that SEV-ES VMs are allowed in SEV-SNP mode

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 23, 2026, Tycho Andersen wrote:
> On Mon, Feb 23, 2026 at 09:15:13AM -0800, Sean Christopherson wrote:
> > On Mon, Feb 23, 2026, Tycho Andersen wrote:
> > > > > +	/*
> > > > > +	 * In some cases when SEV-SNP is enabled, firmware disallows starting
> > > > > +	 * an SEV-ES VM. When SEV-SNP is enabled try to launch an SEV-ES, and
> > > > > +	 * check the underlying firmware error for this case.
> > > > > +	 */
> > > > > +	vm = vm_sev_create_with_one_vcpu(KVM_X86_SEV_ES_VM, guest_sev_es_code,
> > > > > +					 &vcpu);
> > > > 
> > > > If there's a legimate reason why an SEV-ES VM can't be created, then that needs
> > > > to be explicitly enumerated in some way by the kernel.  E.g. is this due to lack
> > > > of ASIDs due to CipherTextHiding or something?
> > > 
> > > Newer firmware that fixes CVE-2025-48514 won't allow SEV-ES VMs to be
> > > started with SNP enabled, there is a footnote (2) about it here:
> > > 
> > > https://www.amd.com/en/resources/product-security/bulletin/amd-sb-3023.html
> > > 
> > > Probably should have included this in the patch, sorry.
> > > 
> > > > Throwing a noodle to see if it sticks is not an option.
> > > 
> > > Sure, we could do some firmware version test to see if it's fixed
> > > instead? Or do this same test in the kernel and export that as an
> > > ioctl?
> > 
> > Uh, no idea what would be ideal, but there absolutely needs to be some way to
> > communicate lack of effective SEV-ES support to userspace, and in a way that
> > doesn't break userspace.
> 
> Just to clarify, by "doesn't break userspace" here you mean that we
> shouldn't revoke the SEV_ES bit from the list of supported VM types
> once we've exposed it? Or you mean preserving the current behavior of
> CPU supports it => bit is set?

I didn't have concrete concerns, I just want to make sure we don't do something
that would confuse userspace and e.g. prevent using KVM for SNP or something.

Hmm, I like the idea of clearing supported_vm_types.  The wrinkle is that "legacy"
deployments that use KVM_SEV_INIT instead of KVM_SEV_INIT2 will use
KVM_X86_DEFAULT_VM, and probably won't check for SEV and SEV_ES VM types.

Alternatively, or in addition to, we could clear X86_FEATURE_SEV_ES.  But clearing
SEV_ES while leaving X86_FEATURE_SEV_SNP makes me nervous.  KVM doesn't *currently*
check for any of those in kvm_cpu_caps, but that could change in the future.  And
it's somewhat misleading, e.g. because sev_snp_guest() expects sev_es_guest() to
be true.

Given that it doesn't make sense for KVM to actively prevent the admin from upgrading
the firmware, I think it's ok if KVM can't "gracefully" handle *every* case.  E.g.
even if KVM clears X86_FEATURE_SEV_ES, userspace could have cached that information
at system boot. 

> > Hrm, I think we also neglected to communicate when SEV and SEV-ES are effectively
> > unusable, e.g. due to CipherTextHiding, so maybe we can kill two birds with one
> > stone?  IIRC, we didn't bother enumerating the limitation with CipherTextHiding
> > because making SEV-ES unusable would require a deliberate act from the admin.
> 
> We know these parameters at module load time so we could unset the
> supported bit, but...
> 
> > "Update firmware" is also an deliberate act, but the side effect of SEV-ES being
> > disabled, not so much.
> 
> since this could be a runtime thing via DOWNLOAD_FIRMWARE_EX at some
> point, I guess we need a new RUNTIME_STATUS ioctl or similar. Then the
> question is: does it live in /dev/sev, or /dev/kvm?

Ugh.  Yeah, updating supported_vm_types definitely seems like the least-awful
option.