Re: [PATCH v2 05/10] KVM: x86: interrupt based APF 'page ready' event delivery

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> On 09/06/20 21:10, Vivek Goyal wrote:
>> Hi Vitaly,
>> 
>> Have a question about page ready events. 
>> 
>> Now we deliver PAGE_NOT_PRESENT page faults only if guest is not in
>> kernel mode. So say kernel tried to access a page and we halted cpu.
>> When page is available, we will inject page_ready interrupt. At
>> that time we don't seem to check whether page_not_present was injected
>> or not. 
>> 
>> IOW, we seem to deliver page_ready irrespective of the fact whether
>> PAGE_NOT_PRESENT was delivered or not. And that means we will be
>> sending page present tokens to guest. Guest will not have a state
>> associated with that token and think that page_not_present has
>> not been delivered yet and allocate an element in hash table for
>> future page_not_present event. And that will lead to memory leak
>> and token conflict etc.
>
> Yes, and this is https://bugzilla.kernel.org/show_bug.cgi?id=208081
> which I was looking at right today.
>

The issue isn't related to the interrupt based APF mechanism, right?
'Page ready' events are always injected (sooner or later). I'll take a
look.

>> While setting up async pf, should we keep track whether associated
>> page_not_present was delivered to guest or not and deliver page_ready
>> accordingly.
>
> Yes, I think so.
>

Something like this? (not even compile tested yet):

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 8e8fea13b6c7..68178d29d35c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1661,7 +1661,7 @@ void kvm_make_scan_ioapic_request(struct kvm *kvm);
 void kvm_make_scan_ioapic_request_mask(struct kvm *kvm,
 				       unsigned long *vcpu_bitmap);
 
-void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
+bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
 				     struct kvm_async_pf *work);
 void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
 				 struct kvm_async_pf *work);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c26dd1363151..e1e840df6b69 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10515,7 +10515,7 @@ bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu)
 	return kvm_arch_interrupt_allowed(vcpu);
 }
 
-void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
+bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
 				     struct kvm_async_pf *work)
 {
 	struct x86_exception fault;
@@ -10532,17 +10532,19 @@ void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
 		fault.address = work->arch.token;
 		fault.async_page_fault = true;
 		kvm_inject_page_fault(vcpu, &fault);
-	} else {
-		/*
-		 * It is not possible to deliver a paravirtualized asynchronous
-		 * page fault, but putting the guest in an artificial halt state
-		 * can be beneficial nevertheless: if an interrupt arrives, we
-		 * can deliver it timely and perhaps the guest will schedule
-		 * another process.  When the instruction that triggered a page
-		 * fault is retried, hopefully the page will be ready in the host.
-		 */
-		kvm_make_request(KVM_REQ_APF_HALT, vcpu);
+		return true;
 	}
+
+	/*
+	 * It is not possible to deliver a paravirtualized asynchronous
+	 * page fault, but putting the guest in an artificial halt state
+	 * can be beneficial nevertheless: if an interrupt arrives, we
+	 * can deliver it timely and perhaps the guest will schedule
+	 * another process.  When the instruction that triggered a page
+	 * fault is retried, hopefully the page will be ready in the host.
+	 */
+	kvm_make_request(KVM_REQ_APF_HALT, vcpu);
+	return false;
 }
 
 void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
@@ -10559,7 +10561,8 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
 		kvm_del_async_pf_gfn(vcpu, work->arch.gfn);
 	trace_kvm_async_pf_ready(work->arch.token, work->cr2_or_gpa);
 
-	if (kvm_pv_async_pf_enabled(vcpu) &&
+	if (work->notpresent_injected &&
+	    kvm_pv_async_pf_enabled(vcpu) &&
 	    !apf_put_user_ready(vcpu, work->arch.token)) {
 		vcpu->arch.apf.pageready_pending = true;
 		kvm_apic_set_irq(vcpu, &irq, NULL);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 802b9e2306f0..2456dc5338f8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -206,6 +206,7 @@ struct kvm_async_pf {
 	unsigned long addr;
 	struct kvm_arch_async_pf arch;
 	bool   wakeup_all;
+	bool notpresent_injected;
 };
 
 void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu);
diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index f1e07fae84e9..de28413abefd 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -189,12 +189,14 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		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);
+	work->notpresent_injected = kvm_arch_async_page_not_present(vcpu, work);
+
+	/* schedule_work() only fails for already queued works */
+	schedule_work(&work->work);
+
 	return 1;
 retry_sync:
 	kvm_put_kvm(work->vcpu->kvm);
@@ -216,6 +218,7 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
 		return -ENOMEM;
 
 	work->wakeup_all = true;
+	work->notpresent_injected = true;
 	INIT_LIST_HEAD(&work->queue); /* for list_del to work */
 
 	spin_lock(&vcpu->async_pf.lock);

-- 
Vitaly