Messages in this thread | | | Subject | Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids | From | Julien Grall <> | Date | Mon, 8 Feb 2021 09:54:13 +0000 |
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On 08/02/2021 09:41, Jürgen Groß wrote: > 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.
I don't think we can easily divide the two because we rely on the IRQ framework to handle the lifecycle of the event. So...
> > 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.
... I don't really see how the difference matter here. The idea is to re-use what's already existing rather than trying to re-invent the wheel with an extra lock (or whatever we can come up).
Cheers,
-- Julien Grall
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