[PATCH v3 3/3] KVM: MMU: consider host cache mode in MMIO page check

[Haozhong Zhang]

Some reserved pages, such as those from NVDIMM DAX devices, are
not for MMIO, and can be mapped with cached memory type for better
performance. However, the above check misconceives those pages as
MMIO.  Because KVM maps MMIO pages with UC memory type, the
performance of guest accesses to those pages would be harmed.
Therefore, we check the host memory type by lookup_memtype() in
addition and only treat UC/UC- pages as MMIO.

Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@xxxxxxxxx>
Reported-by: Kumar, Karthik <karthik.kumar@xxxxxxxxx>
---
 arch/x86/kvm/mmu.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0b481cc9c725..058ff6442638 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2708,7 +2708,23 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
 static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
 {
 	if (pfn_valid(pfn))
-		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
+		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
+			/*
+			 * Some reserved pages, such as those from
+			 * NVDIMM DAX devices, are not for MMIO, and
+			 * can be mapped with cached memory type for
+			 * better performance. However, the above
+			 * check misconceives those pages as MMIO.
+			 * Because KVM maps MMIO pages with UC memory
+			 * type, the performance of guest accesses to
+			 * those pages would be harmed. Therefore, we
+			 * check the host memory type in addition and
+			 * only treat UC/UC- pages as MMIO.
+			 *
+			 * pat_pfn_is_uc() works only when PAT is enabled,
+			 * so check pat_enabled() as well.
+			 */
+			(!pat_enabled() || pat_pfn_is_uc(kvm_pfn_to_pfn(pfn)));
 
 	return true;
 }
-- 
2.14.1