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