[PATCH 3/3] KVM: MMU: consider host cache type in MMIO pfn check

[Haozhong Zhang]

By default, KVM treats a reserved page as for MMIO purpose, and maps
it to guest with UC memory type.  However, some reserved pages are not
for MMIO, such as pages of DAX device (e.g., /dev/daxX.Y). Mapping
them with UC memory type will harm the performance.  In order to
exclude those cases, we check the host cache mode 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 | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0b481cc9c725..d4c821a6df3d 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2707,10 +2707,36 @@ 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));
+	bool is_mmio = true;
 
-	return true;
+	if (pfn_valid(pfn)) {
+		is_mmio = !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
+
+		/*
+		 * By default, KVM treats a reserved page as for MMIO
+		 * purpose, and maps it to guest with UC memory type.
+		 * However, some reserved pages are not for MMIO, such
+		 * as pages of DAX device (e.g., /dev/daxX.Y). Mapping
+		 * them with UC memory type will harm the performance.
+		 * In order to exclude those cases, we check the host
+		 * cache mode in addition and only treat UC/UC- pages
+		 * as MMIO.
+		 *
+		 * track_pfn_insert() works only when PAT is enabled,
+		 * so add pat_enabled() here.
+		 */
+		if (is_mmio && pat_enabled()) {
+			pgprot_t prot;
+			enum page_cache_mode cm;
+
+			track_pfn_insert(NULL, &prot, kvm_pfn_to_pfn(pfn));
+			cm = pgprot2cachemode(prot);
+			is_mmio = (cm == _PAGE_CACHE_MODE_UC ||
+				   cm == _PAGE_CACHE_MODE_UC_MINUS);
+		}
+	}
+
+	return is_mmio;
 }
 
 static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
-- 
2.14.1