KVM: MMU: consider host cache mode in MMIO page check

BugLink: http://bugs.launchpad.net/bugs/1745899
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@intel.com>
Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@intel.com>
Reported-by: Kumar, Karthik <karthik.kumar@intel.com>
Reviewed-by: Xiao Guangrong <xiaoguangrong@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
(cherry picked from commit aa2e063aea7961fb58eafecb924f8818e3d4ecfc)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index e080dbe55360a11e90ea1af53f9457928e1dd665..6e5489d1f511658b78b87f1fb85c7b7592a34c91 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -42,6 +42,7 @@
 #include <linux/kern_levels.h>
 
 #include <asm/page.h>
+#include <asm/pat.h>
 #include <asm/cmpxchg.h>
 #include <asm/io.h>
 #include <asm/vmx.h>
@@ -2708,7 +2709,18 @@ 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_pfn_immune_to_uc_mtrr(pfn));
 
        return true;
 }