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Subject[PATCH 3/3] KVM: MMU: consider host cache type in MMIO pfn check
Date
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@intel.com>
Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@intel.com>
Reported-by: Kumar, Karthik <karthik.kumar@intel.com>
---
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
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