[PATCH v6 2/2] 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 in addition and only treat
UC/UC-/WC 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>
Reviewed-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx>
---
 arch/x86/kvm/mmu.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 89da688784fa..e3b9998b3355 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2708,7 +2708,18 @@ 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, and results in KVM mapping them with UC
+			 * memory type, which would hurt the performance.
+			 * Therefore, we check the host memory type in addition
+			 * and only treat UC/UC-/WC pages as MMIO.
+			 */
+			(!pat_enabled() || pat_immune_to_uc_mtrr(pfn));
 
 	return true;
 }
-- 
2.14.1