KVM: x86: apply guest MTRR virtualization on host reserved pages

Currently guest MTRR is avoided if kvm_is_reserved_pfn returns true.
However, the guest could prefer a different page type than UC for
such pages. A good example is that pass-throughed VGA frame buffer is
not always UC as host expected.

This patch enables full use of virtual guest MTRRs.
Suggested-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Tested-by: Joerg Roedel <jroedel@suse.de> (on AMD)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -1166,14 +1166,11 @@ static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
 	u8 mtrr;
 
 	/*
-	 * 1. MMIO: always map as UC
+	 * 1. MMIO: trust guest MTRR, so same as item 3.
 	 * 2. No passthrough: always map as WB, and force guest PAT to WB as well
 	 * 3. Passthrough: can't guarantee the result, try to trust guest.
 	 */
-	if (is_mmio)
-		return _PAGE_NOCACHE;
-
-	if (!kvm_arch_has_assigned_device(vcpu->kvm))
+	if (!is_mmio && !kvm_arch_has_assigned_device(vcpu->kvm))
 		return 0;
 
 	mtrr = kvm_mtrr_get_guest_memory_type(vcpu, gfn);

arch/x86/kvm/vmx.c

@@ -8632,22 +8632,17 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
 	u64 ipat = 0;
 
 	/* For VT-d and EPT combination
-	 * 1. MMIO: always map as UC
+	 * 1. MMIO: guest may want to apply WC, trust it.
 	 * 2. EPT with VT-d:
 	 *   a. VT-d without snooping control feature: can't guarantee the
-	 *	result, try to trust guest.
+	 *	result, try to trust guest.  So the same as item 1.
 	 *   b. VT-d with snooping control feature: snooping control feature of
 	 *	VT-d engine can guarantee the cache correctness. Just set it
 	 *	to WB to keep consistent with host. So the same as item 3.
 	 * 3. EPT without VT-d: always map as WB and set IPAT=1 to keep
 	 *    consistent with host MTRR
 	 */
-	if (is_mmio) {
-		cache = MTRR_TYPE_UNCACHABLE;
-		goto exit;
-	}
-
-	if (!kvm_arch_has_noncoherent_dma(vcpu->kvm)) {
+	if (!is_mmio && !kvm_arch_has_noncoherent_dma(vcpu->kvm)) {
 		ipat = VMX_EPT_IPAT_BIT;
 		cache = MTRR_TYPE_WRBACK;
 		goto exit;