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

This reverts commit fd717f11015f673487ffc826e59b2bad69d20fe5.
It was reported to cause Machine Check Exceptions (bug 104091).

Reported-by: harn-solo@gmx.de
Cc: stable@vger.kernel.org # 4.2+
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 13 insertions, 5 deletions

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 94b7d15db3fc..9e88078f4146 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1167,11 +1167,14 @@ static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
         u8 mtrr;
 
         /*
-         * 1. MMIO: trust guest MTRR, so same as item 3.
+         * 1. MMIO: always map as UC
          * 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 && !kvm_arch_has_assigned_device(vcpu->kvm))
+        if (is_mmio)
+                return _PAGE_NOCACHE;
+
+        if (!kvm_arch_has_assigned_device(vcpu->kvm))
                 return 0;
 
         if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED) &&
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 64076740251e..06ef4908ba61 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -8617,17 +8617,22 @@ 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: guest may want to apply WC, trust it.
+         * 1. MMIO: always map as UC
          * 2. EPT with VT-d:
          *   a. VT-d without snooping control feature: can't guarantee the
-         *      result, try to trust guest.  So the same as item 1.
+         *      result, try to trust guest.
          *   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 && !kvm_arch_has_noncoherent_dma(vcpu->kvm)) {
+        if (is_mmio) {
+                cache = MTRR_TYPE_UNCACHABLE;
+                goto exit;
+        }
+
+        if (!kvm_arch_has_noncoherent_dma(vcpu->kvm)) {
                 ipat = VMX_EPT_IPAT_BIT;
                 cache = MTRR_TYPE_WRBACK;
                 goto exit;