KVM: SVM: WARN if KVM attempts emulation on #UD or #GP for SEV guests

WARN if KVM attempts to emulate in response to #UD or #GP for SEV guests,
i.e. if KVM intercepts #UD or #GP, as emulation on any fault except #NPF
is impossible since KVM cannot read guest private memory to get the code
stream, and the CPU's DecodeAssists feature only provides the instruction
bytes on #NPF.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <20220120010719.711476-7-seanjc@google.com>
[Warn on EMULTYPE_TRAP_UD_FORCED according to Liam Merwick's review. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -4268,6 +4268,11 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
 	if (!sev_guest(vcpu->kvm))
 		return true;
 
+	/* #UD and #GP should never be intercepted for SEV guests. */
+	WARN_ON_ONCE(emul_type & (EMULTYPE_TRAP_UD |
+				  EMULTYPE_TRAP_UD_FORCED |
+				  EMULTYPE_VMWARE_GP));
+
 	/*
 	 * Emulation is impossible for SEV-ES guests as KVM doesn't have access
 	 * to guest register state.