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

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Message-ID: <bed60b1b671be1e4e2a747b4e6d42ab280e69ab9.camel@redhat.com>
Date: Thu, 19 Dec 2024 10:57:00 -0500
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.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, 2024-12-19 at 07:23 -0800, Sean Christopherson wrote:
> 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;
> 

Ah OK, I missed the "This *will* allow plumbing", that is the fact this this patch
is only a preparation for this.

Best regards,
	Maxim levitsky