KVM: Replace smp_mb() with smp_load_acquire() in the kvm_flush_remote_tlbs()

smp_load_acquire() is enough here and it's cheaper than smp_mb().
Adding a comment about reusing memory barrier of kvm_make_all_cpus_request()
here to keep order between modifications to the page tables and reading mode.
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -191,9 +191,23 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
 #ifndef CONFIG_HAVE_KVM_ARCH_TLB_FLUSH_ALL
 void kvm_flush_remote_tlbs(struct kvm *kvm)
 {
-	long dirty_count = kvm->tlbs_dirty;
+	/*
+	 * Read tlbs_dirty before setting KVM_REQ_TLB_FLUSH in
+	 * kvm_make_all_cpus_request.
+	 */
+	long dirty_count = smp_load_acquire(&kvm->tlbs_dirty);
 
-	smp_mb();
+	/*
+	 * We want to publish modifications to the page tables before reading
+	 * mode. Pairs with a memory barrier in arch-specific code.
+	 * - x86: smp_mb__after_srcu_read_unlock in vcpu_enter_guest
+	 * and smp_mb in walk_shadow_page_lockless_begin/end.
+	 * - powerpc: smp_mb in kvmppc_prepare_to_enter.
+	 *
+	 * There is already an smp_mb__after_atomic() before
+	 * kvm_make_all_cpus_request() reads vcpu->mode. We reuse that
+	 * barrier here.
+	 */
 	if (kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
 		++kvm->stat.remote_tlb_flush;
 	cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);