KVM: x86/mmu: Stop kicking vCPUs to sync the dirty log when PML is disabled

Stop kicking vCPUs in kvm_arch_sync_dirty_log() when PML is disabled.
Kicking vCPUs when PML is disabled serves no purpose and could
negatively impact guest performance.

This restores KVM's behavior to prior to 5.12 commit a018eba ("KVM:
x86: Move MMU's PML logic to common code"), which replaced a
static_call_cond(kvm_x86_flush_log_dirty) with unconditional calls to
kvm_vcpu_kick().

Fixes: a018eba ("KVM: x86: Move MMU's PML logic to common code")
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20231016221228.1348318-1-dmatlack@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: dmatlack

arch/x86/kvm/x86.c

@@ -6355,6 +6355,9 @@ void kvm_arch_sync_dirty_log(struct kvm *kvm, struct kvm_memory_slot *memslot)
 	struct kvm_vcpu *vcpu;
 	unsigned long i;
 
+	if (!kvm_x86_ops.cpu_dirty_log_size)
+		return;
+
 	kvm_for_each_vcpu(i, vcpu, kvm)
 		kvm_vcpu_kick(vcpu);
 }