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

[Tycho Andersen <tycho@kernel.org>]

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?

> 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?

Tycho