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

Julien Grall
2021-02-08T09:34:41Z

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