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Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
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Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
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        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
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        "David S. Miller" <davem@davemloft.net>,
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References: <20210206104932.29064-1-jgross@suse.com>
 <bd63694e-ac0c-7954-ec00-edad05f8da1c@xen.org>
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From:   Julien Grall <julien@xen.org>
Message-ID: <fcf3181b-3efc-55f5-687c-324937b543e6@xen.org>
Date:   Mon, 8 Feb 2021 09:11:18 +0000
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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