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

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 26, 2026, Tycho Andersen wrote:
> On Wed, Feb 25, 2026 at 09:44:15AM -0800, Sean Christopherson wrote:
> > Ya, I don't have a better idea.  Bleeding VM types into the CCP driver might be
> > a bit wonky, though I guess it is uAPI so it's certainly not a KVM-internal detail.
> 
> Turns out this approach breaks the selftests, which is at least one
> userspace:
> 
> # ./sev_init2_tests
> Random seed: 0x6b8b4567
> ==== Test Assertion Failure ====
>   x86/sev_init2_tests.c:141: have_sev_es == !!(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SEV_ES_VM))
>   pid=12498 tid=12498 errno=0 - Success
>      1	0x0000000000402747: main at sev_init2_tests.c:141 (discriminator 2)
>      2	0x00007f9adae2a1c9: ?? ??:0
>      3	0x00007f9adae2a28a: ?? ??:0
>      4	0x0000000000402934: _start at ??:?
>   sev-es: KVM_CAP_VM_TYPES (15) does not match cpuid (checking 8)
> 
> As near as I can tell qemu doesn't do the same anywhere. SNP guests
> run fine, and SEV-ES says something reasonable:
> 
> qemu-system-x86_64: sev_launch_start: LAUNCH_START ret=1 fw_error=21 'Feature not supported'
> qemu-system-x86_64: sev_common_kvm_init: failed to create encryption context
> qemu-system-x86_64: failed to initialize kvm: Operation not permitted
> 
> Thoughts?

Breaking selftests is totally fine, they don't count as real users (the whole
point is to validate KVM behavior; if we weren't allowed to break selftests, we
literally couldn't fix a huge pile of KVM bugs).

Even if a real VMM has a similar sanity check, I wouldn't consider an assertion
firing to be a breaking flaw.  No matter what, the VMM won't be able to launch an
SEV-ES guest.

For selftests, something like this?

	have_sev_es = kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SEV_ES_VM);
	TEST_ASSERT(!have_sev_es || kvm_cpu_has(X86_FEATURE_SEV_ES),
		    "sev-es: SEV_ES_VM supported without SEV_ES in CPUID");

	have_snp = kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SNP_VM);
	TEST_ASSERT(!have_snp || kvm_cpu_has(X86_FEATURE_SEV_SNP),
		    "sev-snp: SNP_VM supported with SEV_SNP in CPUID");