[RFC PATCH 12/28] kvm: mmu: Set tlbs_dirty atomically

[Ben Gardon <bgardon@xxxxxxxxxx>]

The tlbs_dirty mechanism for deferring flushes can be expanded beyond
its current use case. This allows MMU operations which do not
themselves require TLB flushes to notify other threads that there are
unflushed modifications to the paging structure. In order to use this
mechanism concurrently, the updates to the global tlbs_dirty must be
made atomically.

Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
---
 arch/x86/kvm/paging_tmpl.h | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 97903c8dcad16..cc3630c8bd3ea 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -986,6 +986,8 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 	bool host_writable;
 	gpa_t first_pte_gpa;
 	int set_spte_ret = 0;
+	int ret;
+	int tlbs_dirty = 0;
 
 	/* direct kvm_mmu_page can not be unsync. */
 	BUG_ON(sp->role.direct);
@@ -1004,17 +1006,13 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 		pte_gpa = first_pte_gpa + i * sizeof(pt_element_t);
 
 		if (kvm_vcpu_read_guest_atomic(vcpu, pte_gpa, &gpte,
-					       sizeof(pt_element_t)))
-			return 0;
+					       sizeof(pt_element_t))) {
+			ret = 0;
+			goto out;
+		}
 
 		if (FNAME(prefetch_invalid_gpte)(vcpu, sp, &sp->spt[i], gpte)) {
-			/*
-			 * Update spte before increasing tlbs_dirty to make
-			 * sure no tlb flush is lost after spte is zapped; see
-			 * the comments in kvm_flush_remote_tlbs().
-			 */
-			smp_wmb();
-			vcpu->kvm->tlbs_dirty++;
+			tlbs_dirty++;
 			continue;
 		}
 
@@ -1029,12 +1027,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 
 		if (gfn != sp->gfns[i]) {
 			drop_spte(vcpu->kvm, &sp->spt[i]);
-			/*
-			 * The same as above where we are doing
-			 * prefetch_invalid_gpte().
-			 */
-			smp_wmb();
-			vcpu->kvm->tlbs_dirty++;
+			tlbs_dirty++;
 			continue;
 		}
 
@@ -1051,7 +1044,11 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 	if (set_spte_ret & SET_SPTE_NEED_REMOTE_TLB_FLUSH)
 		kvm_flush_remote_tlbs(vcpu->kvm);
 
-	return nr_present;
+	ret = nr_present;
+
+out:
+	xadd(&vcpu->kvm->tlbs_dirty, tlbs_dirty);
+	return ret;
 }
 
 #undef pt_element_t
-- 
2.23.0.444.g18eeb5a265-goog