Re: [PATCH 4/8] KVM: x86: interrupt based APF page-ready event delivery

From: Vivek Goyal <vgoyal@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, x86@kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Jim Mattson <jmattson@google.com>, Gavin Shan <gshan@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/8] KVM: x86: interrupt based APF page-ready event delivery
Date: Thu, 14 May 2020 09:31:57 -0400	[thread overview]
Message-ID: <20200514133157.GB206709@redhat.com> (raw)
In-Reply-To: <87zhabdjlm.fsf@vitty.brq.redhat.com>

On Thu, May 14, 2020 at 10:08:37AM +0200, Vitaly Kuznetsov wrote:
> Vivek Goyal <vgoyal@redhat.com> writes:
> 
> > On Wed, May 13, 2020 at 04:23:55PM +0200, Vitaly Kuznetsov wrote:
> >
> > [..]
> >> >> Also,
> >> >> kdump kernel may not even support APF so it will get very confused when
> >> >> APF events get delivered.
> >> >
> >> > New kernel can just ignore these events if it does not support async
> >> > pf? 
> >> >
> >> > This is somewhat similar to devices still doing interrupts in new
> >> > kernel. And solution for that seemed to be doing a "reset" of devices
> >> > in new kernel. We probably need similar logic where in new kernel
> >> > we simply disable "async pf" so that we don't get new notifications.
> >> 
> >> Right and that's what we're doing - just disabling new notifications.
> >
> > Nice.
> >
> > So why there is a need to deliver "page ready" notifications
> > to guest after guest has disabled async pf. Atleast kdump does not
> > seem to need it. It will boot into second kernel anyway, irrespective
> > of the fact whether it receives page ready or not.
> 
> We don't deliver anything to the guest after it disables APF (neither
> 'page ready' for what was previously missing, nor 'page not ready' for
> new faults), kvm_arch_can_inject_async_page_present() is just another
> misnomer, it should be named something like
> 'kvm_arch_can_unqueue_async_page_present()' meaning that 'page ready'
> notification can be 'unqueued' from internal KVM queue. We will either
> deliver it (when guest has APF enabled) or just drop it (when guest has
> APF disabled). The only case when it has to stay in the queue is when
> guest has APF enabled and the slot is still busy (so it didn't get to
> process a previously delivered notification). We will try to deliver it
> again after guest writes to MSR_KVM_ASYNC_PF_ACK.

This makes sense. Renaming this function to make it more clear will
help understanding code better.

Vivek