Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

[Julien Grall]

On 08/02/2021 09:41, Jürgen Groß wrote:
> On 08.02.21 10:11, Julien Grall wrote:
> > Hi Juergen,
> >
> > On 07/02/2021 12:58, Jürgen Groß wrote:
> > > On 06.02.21 19:46, Julien Grall wrote:
> > > > Hi Juergen,
> > > >
> > > > On 06/02/2021 10:49, Juergen Gross wrote:
> > > > > The first three patches are fixes for XSA-332. The avoid WARN splats
> > > > > and a performance issue with interdomain events.
> > > >
> > > > Thanks for helping to figure out the problem. Unfortunately, I still 
> > > > see reliably the WARN splat with the latest Linux master 
> > > > (1e0d27fce010) + your first 3 patches.
> > > >
> > > > I am using Xen 4.11 (1c7d984645f9) and dom0 is forced to use the 2L 
> > > > events ABI.
> > > >
> > > > After some debugging, I think I have an idea what's went wrong. The 
> > > > problem happens when the event is initially bound from vCPU0 to a 
> > > > different vCPU.
> > > >
> > > >  From the comment in xen_rebind_evtchn_to_cpu(), we are masking the 
> > > > event to prevent it being delivered on an unexpected vCPU. However, 
> > > > I believe the following can happen:
> > > >
> > > > vCPU0                | vCPU1
> > > >                  |
> > > >                  | Call xen_rebind_evtchn_to_cpu()
> > > > receive event X            |
> > > >                  | mask event X
> > > >                  | bind to vCPU1
> > > > <vCPU descheduled>        | unmask event X
> > > >                  |
> > > >                  | receive event X
> > > >                  |
> > > >                  | handle_edge_irq(X)
> > > > handle_edge_irq(X)        |  -> handle_irq_event()
> > > >                  |   -> set IRQD_IN_PROGRESS
> > > >   -> set IRQS_PENDING        |
> > > >                  |   -> evtchn_interrupt()
> > > >                  |   -> clear IRQD_IN_PROGRESS
> > > >                  |  -> IRQS_PENDING is set
> > > >                  |  -> handle_irq_event()
> > > >                  |   -> evtchn_interrupt()
> > > >                  |     -> WARN()
> > > >                  |
> > > >
> > > > All the lateeoi handlers expect a ONESHOT semantic and 
> > > > evtchn_interrupt() is doesn't tolerate any deviation.
> > > >
> > > > I think the problem was introduced by 7f874a0447a9 ("xen/events: fix 
> > > > lateeoi irq acknowledgment") because the interrupt was disabled 
> > > > previously. Therefore we wouldn't do another iteration in 
> > > > handle_edge_irq().
> > >
> > > I think you picked the wrong commit for blaming, as this is just
> > > the last patch of the three patches you were testing.
> >
> > I actually found the right commit for blaming but I copied the 
> > information from the wrong shell :/. The bug was introduced by:
> >
> > c44b849cee8c ("xen/events: switch user event channels to lateeoi model")
> >
> > > > Aside the handlers, I think it may impact the defer EOI mitigation 
> > > > because in theory if a 3rd vCPU is joining the party (let say vCPU A 
> > > > migrate the event from vCPU B to vCPU C). So info->{eoi_cpu, 
> > > > irq_epoch, eoi_time} could possibly get mangled?
> > > >
> > > > For a fix, we may want to consider to hold evtchn_rwlock with the 
> > > > write permission. Although, I am not 100% sure this is going to 
> > > > prevent everything.
> > >
> > > It will make things worse, as it would violate the locking hierarchy
> > > (xen_rebind_evtchn_to_cpu() is called with the IRQ-desc lock held).
> >
> > Ah, right.
> >
> > > On a first glance I think we'll need a 3rd masking state ("temporarily
> > > masked") in the second patch in order to avoid a race with lateeoi.
> > >
> > > In order to avoid the race you outlined above we need an "event is being
> > > handled" indicator checked via test_and_set() semantics in
> > > handle_irq_for_port() and reset only when calling clear_evtchn().
> >
> > It feels like we are trying to workaround the IRQ flow we are using 
> > (i.e. handle_edge_irq()).
>
> I'm not really sure this is the main problem here. According to your
> analysis the main problem is occurring when handling the event, not when
> handling the IRQ: the event is being received on two vcpus.

I don't think we can easily divide the two because we rely on the IRQ 
framework to handle the lifecycle of the event. So...

> Our problem isn't due to the IRQ still being pending, but due it being
> raised again, which should happen for a one shot IRQ the same way.

... I don't really see how the difference matter here. The idea is to 
re-use what's already existing rather than trying to re-invent the wheel 
with an extra lock (or whatever we can come up).

Cheers,

--
Julien Grall