Re: [PATCH] KVM: x86: Don't patch L2's hypercall if L1 intercepts #UD

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 12, 2026, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> The fix_hypercall_test selftest fails when run in an L1 VM on an AMD host:
> 
>   ret == (u64)-EFAULT
> 
> On SVM, L0 always intercepts #UD in vmcb01, so nested_svm_exit_special()
> claims every #UD from L2 for L0, before L1's intercepts are consulted.
> L0 then emulates L2's VMCALL and, based on the quirk that L0 userspace
> configured for L1, patches it to VMMCALL. This ignores L1's decision to
> disable KVM_X86_QUIRK_FIX_HYPERCALL_INSN for L2.
> 
> Synthesize #UD instead if the vCPU is in L2 and L1 intercepts #UD, so
> the nested exception machinery reflects it to L1.  Keep patching L2's
> instruction when L1 doesn't intercept #UD, as existing userspace relies
> on it, e.g. nested SVM selftests execute VMCALL in L2 and expect L1 to
> see SVM_EXIT_VMMCALL.
> 
> Fixes: f1a9761fbb00 ("KVM: x86: Allow userspace to opt out of hypercall patching")
> Signed-off-by: Like Xu <likexu@tencent.com>

I think I'd rather fix this by skipping the selftest, e.g.

diff --git a/tools/testing/selftests/kvm/x86/fix_hypercall_test.c b/tools/testing/selftests/kvm/x86/fix_hypercall_test.c
index f11fcc520f24..bb2fdd622538 100644
--- a/tools/testing/selftests/kvm/x86/fix_hypercall_test.c
+++ b/tools/testing/selftests/kvm/x86/fix_hypercall_test.c
@@ -135,6 +135,7 @@ KVM_ONE_VCPU_TEST(fix_hypercall, disable_quirk, guest_main)
 
 int main(int argc, char *argv[])
 {
+	TEST_REQUIRE(!this_cpu_has(X86_FEATURE_HYPERVISOR));
 	TEST_REQUIRE(kvm_check_cap(KVM_CAP_DISABLE_QUIRKS2) & KVM_X86_QUIRK_FIX_HYPERCALL_INSN);
 
 	return test_harness_run(argc, argv);

I doubt there's a deployment that would actually break if L0 stopped patching L2,
but the right solution here really is to disable the quirk in L0.  Any risk of
breaking existing setups, however small, just isn't worth it IMO.