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

Jürgen Groß
2021-02-08T09:45:12Z

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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.

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.

But maybe I'm misunderstanding your idea.

Juergen

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