Patch "Revert "KVM: x86: apply guest MTRR virtualization on host reserved pages"" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
revert-kvm-x86-apply-guest-mtrr-virtualization-on-host-reserved-pages.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paolo Bonzini <[email protected]>
Date: Thu, 1 Oct 2015 13:12:47 +0200
Subject: Revert "KVM: x86: apply guest MTRR virtualization on host reserved pages"

From: Paolo Bonzini <[email protected]>

commit 606decd67049217684e3cb5a54104d51ddd4ef35 upstream.

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

---
 arch/x86/kvm/svm.c |    7 +++++--
 arch/x86/kvm/vmx.c |   11 ++++++++---
 2 files changed, 13 insertions(+), 5 deletions(-)

@@ -1166,11 +1166,14 @@ static u64 svm_get_mt_mask(struct kvm_vc
 	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;
 
 	mtrr = kvm_mtrr_get_guest_memory_type(vcpu, gfn);

@@ -8634,17 +8634,22 @@ static u64 vmx_get_mt_mask(struct kvm_vc
 	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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/kvm-fix-double-free-for-fast-mmio-eventfd.patch queue-4.2/kvm-fix-zero-length-mmio-searching.patch queue-4.2/revert-kvm-x86-apply-guest-mtrr-virtualization-on-host-reserved-pages.patch queue-4.2/kvm-vmx-fix-vpid-is-0000h-in-non-root-operation.patch queue-4.2/revert-kvm-svm-use-npt-page-attributes.patch queue-4.2/revert-kvm-svm-sync-g_pat-with-guest-written-pat-value.patch queue-4.2/kvm-factor-out-core-eventfd-assign-deassign-logic.patch queue-4.2/kvm-x86-trap-amd-msrs-for-the-tseg-base-and-mask.patch queue-4.2/kvm-svm-reset-mmu-on-vcpu-reset.patch queue-4.2/kvm-don-t-try-to-register-to-kvm_fast_mmio_bus-for-non-mmio-eventfd.patch