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

[Julien Grall <julien@xxxxxxx>]

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()).
This reminds me the thread we had before discovering XSA-332 (see [1]). 
Back then, it was suggested to switch back to handle_fasteoi_irq().
Cheers,

[1] 
https://lore.kernel.org/xen-devel/alpine.DEB.2.21.2004271552430.29217@sstabellini-ThinkPad-T480s/
--
Julien Grall