KVM: async_pf: Cleanup kvm_setup_async_pf()

schedule_work() returns 'false' only when the work is already on the queue
and this can't happen as kvm_setup_async_pf() always allocates a new one.
Also, to avoid potential race, it makes sense to to schedule_work() at the
very end after we've added it to the queue.

While on it, do some minor cleanup. gfn_to_pfn_async() mentioned in a
comment does not currently exist and, moreover, we can check
kvm_is_error_hva() at the very beginning, before we try to allocate work so
'retry_sync' label can go away completely.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200610175532.779793-1-vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/async_pf.c

@@ -164,7 +164,9 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	if (vcpu->async_pf.queued >= ASYNC_PF_PER_VCPU)
 		return 0;
 
-	/* setup delayed work */
+	/* Arch specific code should not do async PF in this case */
+	if (unlikely(kvm_is_error_hva(hva)))
+		return 0;
 
 	/*
 	 * do alloc nowait since if we are going to sleep anyway we
@@ -183,24 +185,15 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	mmget(work->mm);
 	kvm_get_kvm(work->vcpu->kvm);
 
-	/* this can't really happen otherwise gfn_to_pfn_async
-	   would succeed */
-	if (unlikely(kvm_is_error_hva(work->addr)))
-		goto retry_sync;
-
 	INIT_WORK(&work->work, async_pf_execute);
-	if (!schedule_work(&work->work))
-		goto retry_sync;
 
 	list_add_tail(&work->queue, &vcpu->async_pf.queue);
 	vcpu->async_pf.queued++;
 	kvm_arch_async_page_not_present(vcpu, work);
+
+	schedule_work(&work->work);
+
 	return 1;
-retry_sync:
-	kvm_put_kvm(work->vcpu->kvm);
-	mmput(work->mm);
-	kmem_cache_free(async_pf_cache, work);
-	return 0;
 }
 
 int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)