KVM: MMU: Explicitly set D-bit for writable spte.

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 <[email protected]>
Reviewed-by: Xiao Guangrong <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: Kai Huang

arch/x86/kvm/mmu.c

@@ -2597,8 +2597,10 @@ 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);
+		spte |= shadow_dirty_mask;
+	}
 
 set_pte:
 	if (mmu_spte_update(sptep, spte))
@@ -2914,6 +2916,18 @@ 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 unnecessary PML logging. See comments in
+	 * set_spte. But fast_page_fault is very unlikely to happen with PML
+	 * enabled, so we do not do this. 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.
+	 *
+	 * Compare with set_spte where instead shadow_dirty_mask is set.
+	 */
 	if (cmpxchg64(sptep, spte, spte | PT_WRITABLE_MASK) == spte)
 		mark_page_dirty(vcpu->kvm, gfn);