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>

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;

arch/x86/kvm/mmu/spte.c

@@ -19,7 +19,7 @@
 #include <asm/memtype.h>
 #include <asm/vmx.h>
 
-static bool __read_mostly enable_mmio_caching = true;
+bool __read_mostly enable_mmio_caching = true;
 module_param_named(mmio_caching, enable_mmio_caching, bool, 0444);
 
 u64 __read_mostly shadow_host_writable_mask;
@@ -351,6 +351,9 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
 	    WARN_ON(mmio_value && (REMOVED_SPTE & mmio_mask) == mmio_value))
 		mmio_value = 0;
 
+	if (!mmio_value)
+		enable_mmio_caching = false;
+
 	shadow_mmio_value = mmio_value;
 	shadow_mmio_mask  = mmio_mask;
 	shadow_mmio_access_mask = access_mask;

arch/x86/kvm/mmu/spte.h

@@ -5,6 +5,8 @@
 
 #include "mmu_internal.h"
 
+extern bool __read_mostly enable_mmio_caching;
+
 /*
  * A MMU present SPTE is backed by actual memory and may or may not be present
  * in hardware.  E.g. MMIO SPTEs are not considered present.  Use bit 11, as it
@@ -204,7 +206,7 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
 static inline bool is_mmio_spte(u64 spte)
 {
 	return (spte & shadow_mmio_mask) == shadow_mmio_value &&
-	       likely(shadow_mmio_value);
+	       likely(enable_mmio_caching);
 }
 
 static inline bool is_shadow_present_pte(u64 pte)