KVM: SVM: Explicitly require DECODEASSISTS to enable SEV support

Add a sanity check on DECODEASSIST being support if SEV is supported, as
KVM cannot read guest private memory and thus relies on the CPU to
provide the instruction byte stream on #NPF for emulation.  The intent of
the check is to document the dependency, it should never fail in practice
as producing hardware that supports SEV but not DECODEASSISTS would be
non-sensical.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <20220120010719.711476-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 6a22798eaaee9d50ed2f49137d3a56fe67316fda..17b53457d866416a15cf872ac40169bd0f9230fd 100644 (file)
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2100,8 +2100,13 @@ void __init sev_hardware_setup(void)
        if (!sev_enabled || !npt_enabled)
                goto out;
 
-       /* Does the CPU support SEV? */
-       if (!boot_cpu_has(X86_FEATURE_SEV))
+       /*
+        * SEV must obviously be supported in hardware.  Sanity check that the
+        * CPU supports decode assists, which is mandatory for SEV guests to
+        * support instruction emulation.
+        */
+       if (!boot_cpu_has(X86_FEATURE_SEV) ||
+           WARN_ON_ONCE(!boot_cpu_has(X86_FEATURE_DECODEASSISTS)))
                goto out;
 
        /* Retrieve SEV CPUID information */