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
