linux-kernel - Re: [PATCH 09/20] KVM: selftests: Honor "stop" request in dirty ring test

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Message-ID: <Z2Q6jK1E0KfX7n7l@google.com>
Date: Thu, 19 Dec 2024 07:23:56 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 09/20] KVM: selftests: Honor "stop" request in dirty ring test

On Thu, Dec 19, 2024, Maxim Levitsky wrote:
> On Wed, 2024-12-18 at 18:00 -0800, Sean Christopherson wrote:
> > On Tue, Dec 17, 2024, Maxim Levitsky wrote:
> > > On Fri, 2024-12-13 at 17:07 -0800, Sean Christopherson wrote:
> > > > Now that the vCPU doesn't dirty every page on the first iteration for
> > > > architectures that support the dirty ring, honor vcpu_stop in the dirty
> > > > ring's vCPU worker, i.e. stop when the main thread says "stop".  This will
> > > > allow plumbing vcpu_stop into the guest so that the vCPU doesn't need to
> > > > periodically exit to userspace just to see if it should stop.
> > > 
> > > This is very misleading - by the very nature of this test it all runs in
> > > userspace, so every time KVM_RUN ioctl exits, it is by definition an
> > > userspace VM exit.
> > 
> > I honestly don't see how being more precise is misleading.
> 
> "Exit to userspace" is misleading - the *whole test* is userspace.

No, the test has a guest component.  Just because the host portion of the test
only runs in userspace doesn't make KVM go away.  If this were pure emulation,
then I would completely agree, but there multiple distinct components here, one
of which is host userspace.

> You treat vCPU worker thread as if it not userspace, but it is *userspace* by
> the definition of how KVM works.

By simply "vCPU" I am strictly referring to the guest entity.  I refered to the
host side worker as "vCPU woker" to try to distinguish between the two.

> Right way to say it is something like 'don't pause the vCPU worker thread
> when its not needed' or something like that.

That's inaccurate though.  GUEST_SYNC() doesn't pause the vCPU, it forces it to
exit to userspace.  The test forces the vCPU to exit to check to see if it needs
to pause/stop, which I'm contending is wasteful and unnecessarily complex.  The
vCPU can instead check to see if it needs to stop simply by reading the global
variable.

If vcpu_sync_stop_requested is false, the worker thread immediated resumes the
vCPU.

  /* Should only be called after a GUEST_SYNC */
  static void vcpu_handle_sync_stop(void)
  {
	if (atomic_read(&vcpu_sync_stop_requested)) {
		/* It means main thread is sleeping waiting */
		atomic_set(&vcpu_sync_stop_requested, false);
		sem_post(&sem_vcpu_stop);
		sem_wait_until(&sem_vcpu_cont);
	}
  }

The future cleanup is to change the guest loop to keep running _in the guest_
until a stop is requested.  Whereas the current code exits to userspace every
4096 writes to see if it should stop.  But as above, the vCPU doesn't actually
stop on each exit.

@@ -112,7 +111,7 @@ static void guest_code(void)
 #endif

 	while (true) {
-		for (i = 0; i < TEST_PAGES_PER_LOOP; i++) {
+		while (!READ_ONCE(vcpu_stop)) {
 			addr = guest_test_virt_mem;
 			addr += (guest_random_u64(&guest_rng) % guest_num_pages)
 				* guest_page_size;