KVM: x86: Defer MMU sync on PCID invalidation

Defer the MMU sync on PCID invalidation so that multiple sync requests in
a single VM-Exit are batched.  This is a very minor optimization as
checking for unsync'd children is quite cheap.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210609234235.1244004-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -1074,7 +1074,7 @@ static void kvm_invalidate_pcid(struct kvm_vcpu *vcpu, unsigned long pcid)
 	 * happen anyway before switching to any other CR3.
 	 */
 	if (kvm_get_active_pcid(vcpu) == pcid) {
-		kvm_mmu_sync_roots(vcpu);
+		kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
 		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 	}