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().
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.
Does my write-up make sense to you?
Cheers,