On 11/03/2017 04:51 PM, Haozhong Zhang wrote:
On 11/03/17 14:54 +0800, Xiao Guangrong wrote:
On 11/03/2017 01:53 PM, Haozhong Zhang wrote:
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 by lookup_memtype() 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 | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0b481cc9c725..e9ed0e666a83 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2708,7 +2708,24 @@ 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.
+ * 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- pages as MMIO.
+ *
+ * pat_pfn_is_uc() works only when PAT is enabled,
+ * so check pat_enabled() as well.
+ */
+ (!pat_enabled() ||
+ pat_pfn_is_uc(kvm_pfn_t_to_pfn_t(pfn)));
Can it be compiled if !CONFIG_PAT?
Yes.
What I check via pat_enabled() is not only whether PAT support is
compiled, but also whether PAT is enabled at runtime.
The issue is about pat_pfn_is_uc() which is implemented only if CONFIG_PAT is
enabled, but you used it here unconditionally.
I am not sure if gcc is smart enough to omit pat_pfn_is_uc() completely under
this case. If you really have done the test to compile kernel and KVM module
with CONFIG_PAT disabled, it is fine.
