KVM: x86/mmu: Check for host MMIO exclusion from mem encrypt iff necessary

When determining whether or not a SPTE needs to have SME/SEV's memory
encryption flag set, do the moderately expensive host MMIO pfn check if
and only if the memory encryption mask is non-zero.

Note, KVM could further optimize the host MMIO checks by making a single
call to kvm_is_mmio_pfn(), but the tdp_enabled path (for EPT's memtype
handling) will likely be split out to a separate flow[*].  At that point,
a better approach would be to shove the call to kvm_is_mmio_pfn() into
VMX code so that AMD+NPT without SME doesn't get hit with an unnecessary
lookup.

[*] https://lkml.kernel.org/r/20220321224358.1305530-3-bgardon@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220415004909.2216670-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/spte.c

@@ -139,7 +139,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	else
 		pte_access &= ~ACC_WRITE_MASK;
 
-	if (!kvm_is_mmio_pfn(pfn))
+	if (shadow_me_mask && !kvm_is_mmio_pfn(pfn))
 		spte |= shadow_me_mask;
 
 	spte |= (u64)pfn << PAGE_SHIFT;