Re: [PATCH] acpi: pcihp: make pending delete expire in 5sec

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 4 Apr 2023 08:40:45 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Apr 04, 2023 at 01:06:38PM +0530, Ani Sinha wrote:
> > 
> > 
> > On Tue, 4 Apr 2023, Gerd Hoffmann wrote:
> >   
> > >   Hi,
> > >  
> > > > > Allowing pending delete expire brings ACPI PCI hotplug on par
> > > > > with native PCIe unplug behavior [1] which in its turn refers
> > > > > back to ACPI PCI hotplug ability to repeat unplug requests.  
> > >  
> > > > A bit concerned about how this interacts with failover,
> > > > and 5sec is a lot of time that I hoped we'd avoid with acpi.
> > > > Any better ideas of catching such misbehaving guests?  
> > >
> > > The 5sec are coming from the pcie spec: The hot-unplug request can be
> > > canceled within 5 seconds by pressing the button again. The problem here
> > > is that both hotplug and hot-unplug use the same signaling path, so we
> > > really have to wait the 5 seconds to avoid the OS mis-interpreting the
> > > button press as 'cancel' event.
> > >
> > > ACPI hotplug hasn't this problem.  A unplug request is a unplug request,  
> > 
> > For ACPI case, I think all we want is to make sure that the first unplug
> > event to not stick forever. A non-zero but small delay would make sure
> > that the first
> > unplug event would get cleared after that interval and subsequent unplug
> > events will get registered without that error.
> >   
> > > period.  And it can't be canceled.  So it should be possible to use a
> > > shorter period.  Possibly even no delay at all.
> > >
> > > take care,
> > >   Gerd
> > >
> > >  
> 
> 
> But why do we want a delay at all? for acpi you can resend
> the interrupt as many times as you like.

yep, we can. It makes possible for user to cause limited
"interrupt storm". That also leads to device_del abuse [1]

1) https://www.mail-archive.com/qemu-devel@nongnu.org/msg952738.html