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

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().


[1] https://lore.kernel.org/xen-devel/alpine.DEB.2.21.2004271552430.29217@sstabellini-ThinkPad-T480s/

Julien Grall



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