Messages in this thread | | | From | Jürgen Groß <> | Subject | Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids | Date | Sun, 7 Feb 2021 13:58:20 +0100 |
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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 [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |