KVM: x86/mmu: Use enable_mmio_caching to track if MMIO caching is enabled

Clear enable_mmio_caching if hardware can't support MMIO caching and use
the dedicated flag to detect if MMIO caching is enabled instead of
assuming shadow_mmio_value==0 means MMIO caching is disabled.  TDX will
use a zero value even when caching is enabled, and is_mmio_spte() isn't
so hot that it needs to avoid an extra memory access, i.e. there's no
reason to be super clever.  And the clever approach may not even be more
performant, e.g. gcc-11 lands the extra check on a non-zero value inline,
but puts the enable_mmio_caching out-of-line, i.e. avoids the few extra
uops for non-MMIO SPTEs.

Cc: Isaku Yamahata <isaku.yamahata@intel.com>
Cc: Kai Huang <kai.huang@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220420002747.3287931-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 77785587332e..7b088417d978 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3036,7 +3036,7 @@ static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
 		 * and only if L1's MAXPHYADDR is inaccurate with respect to
 		 * the hardware's).
 		 */
-		if (unlikely(!shadow_mmio_value) ||
+		if (unlikely(!enable_mmio_caching) ||
 		    unlikely(fault->gfn > kvm_mmu_max_gfn())) {
 			*ret_val = RET_PF_EMULATE;
 			return true;