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

Jürgen Groß
2021-02-07T12:59:18Z

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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.

> 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).

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().

> Does my write-up make sense to you?

Yes. What about my reply? ;-)

Juergen

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