KVM: MTRR: treat memory as writeback if MTRR is disabled in guest CPUID

Virtual machines can be run with CPUID such that there are no MTRRs.
In that case, the firmware will never enable MTRRs and it is obviously
undesirable to run the guest entirely with UC memory.  Check out guest
CPUID, and use WB memory if MTRR do not exist.

Cc: qemu-stable@nongnu.org
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=107561
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.h

@@ -38,6 +38,14 @@ static inline bool guest_cpuid_has_xsave(struct kvm_vcpu *vcpu)
 	return best && (best->ecx & bit(X86_FEATURE_XSAVE));
 }
 
+static inline bool guest_cpuid_has_mtrr(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpuid_entry2 *best;
+
+	best = kvm_find_cpuid_entry(vcpu, 1, 0);
+	return best && (best->edx & bit(X86_FEATURE_MTRR));
+}
+
 static inline bool guest_cpuid_has_tsc_adjust(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;

arch/x86/kvm/mtrr.c

@@ -120,14 +120,22 @@ static u8 mtrr_default_type(struct kvm_mtrr *mtrr_state)
 	return mtrr_state->deftype & IA32_MTRR_DEF_TYPE_TYPE_MASK;
 }
 
-static u8 mtrr_disabled_type(void)
+static u8 mtrr_disabled_type(struct kvm_vcpu *vcpu)
 {
 	/*
 	 * Intel SDM 11.11.2.2: all MTRRs are disabled when
 	 * IA32_MTRR_DEF_TYPE.E bit is cleared, and the UC
 	 * memory type is applied to all of physical memory.
+	 *
+	 * However, virtual machines can be run with CPUID such that
+	 * there are no MTRRs.  In that case, the firmware will never
+	 * enable MTRRs and it is obviously undesirable to run the
+	 * guest entirely with UC memory and we use WB.
 	 */
-	return MTRR_TYPE_UNCACHABLE;
+	if (guest_cpuid_has_mtrr(vcpu))
+		return MTRR_TYPE_UNCACHABLE;
+	else
+		return MTRR_TYPE_WRBACK;
 }
 
 /*
@@ -675,7 +683,7 @@ u8 kvm_mtrr_get_guest_memory_type(struct kvm_vcpu *vcpu, gfn_t gfn)
 	}
 
 	if (iter.mtrr_disabled)
-		return mtrr_disabled_type();
+		return mtrr_disabled_type(vcpu);
 
 	/* not contained in any MTRRs. */
 	if (type == -1)