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 09:11:18 +0000 |
| |
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
|  |