This patch avoids unnecessary dirty GPA logging to PML buffer in EPT violation
path by setting D-bit manually prior to the occurrence of the write from guest.
We only set D-bit manually in set_spte, and leave fast_page_fault path
unchanged, as fast_page_fault is very unlikely to happen in case of PML.
For the hva <-> pa change case, the spte is updated to either read-only (host
pte is read-only) or be dropped (host pte is writeable), and both cases will be
handled by above changes, therefore no change is necessary.
Signed-off-by: Kai Huang <kai.huang@xxxxxxxxxxxxxxx>
---
arch/x86/kvm/mmu.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index c438224..fb35535 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2597,8 +2597,14 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
}
}
- if (pte_access & ACC_WRITE_MASK)
+ if (pte_access & ACC_WRITE_MASK) {
mark_page_dirty(vcpu->kvm, gfn);
+ /*
+ * Explicitly set dirty bit. It is used to eliminate unnecessary
+ * dirty GPA logging in case of PML is enabled on VMX.
+ */
+ spte |= shadow_dirty_mask;
+ }
set_pte:
if (mmu_spte_update(sptep, spte))
@@ -2914,6 +2920,16 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
*/
gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->spt);
+ /*
+ * Theoretically we could also set dirty bit (and flush TLB) here in
+ * order to eliminate the unnecessary PML logging. See comments in
+ * set_spte. But as in case of PML, fast_page_fault is very unlikely to
+ * happen so we leave it unchanged. This might result in the same GPA
+ * to be logged in PML buffer again when the write really happens, and
+ * eventually to be called by mark_page_dirty twice. But it's also no
+ * harm. This also avoids the TLB flush needed after setting dirty bit
+ * so non-PML cases won't be impacted.
+ */
if (cmpxchg64(sptep, spte, spte | PT_WRITABLE_MASK) == spte)
mark_page_dirty(vcpu->kvm, gfn);
--
2.1.0
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