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

Web:        https://git.kernel.org/torvalds/c/aa2e063aea7961fb58eafecb924f8818e3d4ecfc
Commit:     aa2e063aea7961fb58eafecb924f8818e3d4ecfc
Parent:     b8d7044bcff7a955257b242515bcf1e5045edd9b
Refname:    refs/heads/master
Author:     Haozhong Zhang <[email protected]>
AuthorDate: Wed Dec 20 15:29:29 2017 +0800
Committer:  Radim Krčmář <[email protected]>
CommitDate: Tue Jan 16 16:40:09 2018 +0100

    KVM: MMU: consider host cache mode in MMIO page check
    
    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 <[email protected]>
    Reported-by: Cuevas Escareno, Ivan D <[email protected]>
    Reported-by: Kumar, Karthik <[email protected]>
    Reviewed-by: Xiao Guangrong <[email protected]>
    Signed-off-by: Paolo Bonzini <[email protected]>
    Signed-off-by: Radim Krčmář <[email protected]>
---
 arch/x86/kvm/mmu.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index ff1e9ee259cf..1f1da400fcde 100644
--- 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;
 }
--
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