KVM: x86/mmu: Restrict mapping level based on guest MTRR iff they're used

Restrict the mapping level for SPTEs based on the guest MTRRs if and only
if KVM may actually use the guest MTRRs to compute the "real" memtype.
For all forms of paging, guest MTRRs are purely virtual in the sense that
they are completely ignored by hardware, i.e. they affect the memtype
only if software manually consumes them.  The only scenario where KVM
consumes the guest MTRRs is when shadow_memtype_mask is non-zero and the
guest has non-coherent DMA, in all other cases KVM simply leaves the PAT
field in SPTEs as '0' to encode WB memtype.

Note, KVM may still ultimately ignore guest MTRRs, e.g. if the backing
pfn is host MMIO, but false positives are ok as they only cause a slight
performance blip (unless the guest is doing weird things with its MTRRs,
which is extremely unlikely).
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220715230016.3762909-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -4296,14 +4296,26 @@ EXPORT_SYMBOL_GPL(kvm_handle_page_fault);
 
 int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
-	while (fault->max_level > PG_LEVEL_4K) {
-		int page_num = KVM_PAGES_PER_HPAGE(fault->max_level);
-		gfn_t base = (fault->addr >> PAGE_SHIFT) & ~(page_num - 1);
-
-		if (kvm_mtrr_check_gfn_range_consistency(vcpu, base, page_num))
-			break;
+	/*
+	 * If the guest's MTRRs may be used to compute the "real" memtype,
+	 * restrict the mapping level to ensure KVM uses a consistent memtype
+	 * across the entire mapping.  If the host MTRRs are ignored by TDP
+	 * (shadow_memtype_mask is non-zero), and the VM has non-coherent DMA
+	 * (DMA doesn't snoop CPU caches), KVM's ABI is to honor the memtype
+	 * from the guest's MTRRs so that guest accesses to memory that is
+	 * DMA'd aren't cached against the guest's wishes.
+	 *
+	 * Note, KVM may still ultimately ignore guest MTRRs for certain PFNs,
+	 * e.g. KVM will force UC memtype for host MMIO.
+	 */
+	if (shadow_memtype_mask && kvm_arch_has_noncoherent_dma(vcpu->kvm)) {
+		for ( ; fault->max_level > PG_LEVEL_4K; --fault->max_level) {
+			int page_num = KVM_PAGES_PER_HPAGE(fault->max_level);
+			gfn_t base = (fault->addr >> PAGE_SHIFT) & ~(page_num - 1);
 
-		--fault->max_level;
+			if (kvm_mtrr_check_gfn_range_consistency(vcpu, base, page_num))
+				break;
+		}
 	}
 
 	return direct_page_fault(vcpu, fault);