[PATCH 1/2] kvm: x86/mmu: Reduce the update to the spte in FNAME(sync_page)

[Lai Jiangshan]

From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>

Sometimes when the guest updates its pagetable, it adds only new gptes
to it without changing any existed one, so there is no point to update
the sptes for these existed gptes.

Also when the sptes for these unchanged gptes are updated, the AD
bits are also removed since make_spte() is called with prefetch=true
which might result unneeded TLB flushing.

Do nothing if the permissions are unchanged or only write-access is
being added.  Only update the spte when write-access is being removed.
Drop the SPTE otherwise.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
---
 arch/x86/kvm/mmu/paging_tmpl.h | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index e5662dbd519c..613f043a3e9e 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -1023,7 +1023,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 	for (i = 0; i < SPTE_ENT_PER_PAGE; i++) {
 		u64 *sptep, spte;
 		struct kvm_memory_slot *slot;
-		unsigned pte_access;
+		unsigned old_pte_access, pte_access;
 		pt_element_t gpte;
 		gpa_t pte_gpa;
 		gfn_t gfn;
@@ -1064,6 +1064,23 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 			continue;
 		}
 
+		/*
+		 * Drop the SPTE if the new protections would result in access
+		 * permissions other than write-access is changing.  Do nothing
+		 * if the permissions are unchanged or only write-access is
+		 * being added.  Only update the spte when write-access is being
+		 * removed.
+		 */
+		old_pte_access = kvm_mmu_page_get_access(sp, i);
+		if (old_pte_access == pte_access ||
+		    (old_pte_access | ACC_WRITE_MASK) == pte_access)
+			continue;
+		if (old_pte_access != (pte_access | ACC_WRITE_MASK)) {
+			drop_spte(vcpu->kvm, &sp->spt[i]);
+			flush = true;
+			continue;
+		}
+
 		/* Update the shadowed access bits in case they changed. */
 		kvm_mmu_page_set_access(sp, i, pte_access);
 
-- 
2.19.1.6.gb485710b