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To:     Julien Grall <julien@xen.org>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        netdev@vger.kernel.org, linux-scsi@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
        Jens Axboe <axboe@kernel.dk>, Wei Liu <wei.liu@kernel.org>,
        Paul Durrant <paul@xen.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
References: <20210206104932.29064-1-jgross@suse.com>
 <bd63694e-ac0c-7954-ec00-edad05f8da1c@xen.org>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
Message-ID: <eeb62129-d9fc-2155-0e0f-aff1fbb33fbc@suse.com>
Date:   Sun, 7 Feb 2021 13:58:20 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
To: Julien Grall <julien@xen.org>, xen-devel@lists.xenproject.org,
 linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
 netdev@vger.kernel.org, linux-scsi@vger.kernel.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>, stable@vger.kernel.org,
 Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
 =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
 Jens Axboe <axboe@kernel.dk>, Wei Liu <wei.liu@kernel.org>,
 Paul Durrant <paul@xen.org>, "David S. Miller" <davem@davemloft.net>,
 Jakub Kicinski <kuba@kernel.org>
Message-ID: <eeb62129-d9fc-2155-0e0f-aff1fbb33fbc@suse.com>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
References: <20210206104932.29064-1-jgross@suse.com>
 <bd63694e-ac0c-7954-ec00-edad05f8da1c@xen.org>
In-Reply-To: <bd63694e-ac0c-7954-ec00-edad05f8da1c@xen.org>

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On 06.02.21 19:46, Julien Grall wrote:
> Hi Juergen,
>=20
> 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.
>=20
> Thanks for helping to figure out the problem. Unfortunately, I still se=
e=20
> reliably the WARN splat with the latest Linux master (1e0d27fce010) +=20
> your first 3 patches.
>=20
> I am using Xen 4.11 (1c7d984645f9) and dom0 is forced to use the 2L=20
> events ABI.
>=20
> After some debugging, I think I have an idea what's went wrong. The=20
> problem happens when the event is initially bound from vCPU0 to a=20
> different vCPU.
>=20
>  From the comment in xen_rebind_evtchn_to_cpu(), we are masking the=20
> event to prevent it being delivered on an unexpected vCPU. However, I=20
> believe the following can happen:
>=20
> vCPU0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0 | vCPU1
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 | Call xen_rebind_evtchn_to_cpu()
> receive event X=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0 |
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 | mask event X
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 | bind to vCPU1
> <vCPU descheduled>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | unmask e=
vent X
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 | receive event X
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 | handle_edge_irq(X)
> handle_edge_irq(X)=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0 ->=
 handle_irq_event()
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0 -> set IRQD_IN_PROGRESS
>  =C2=A0-> set IRQS_PENDING=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0 -> evtchn_interrupt()
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0 -> clear IRQD_IN_PROGRESS
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0 -> IRQS_PENDING is set
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0 -> handle_irq_event()
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0 -> evtchn_interrupt()
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |=C2=A0=C2=A0=C2=A0=C2=A0 -> WARN()
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0 |
>=20
> All the lateeoi handlers expect a ONESHOT semantic and=20
> evtchn_interrupt() is doesn't tolerate any deviation.
>=20
> I think the problem was introduced by 7f874a0447a9 ("xen/events: fix=20
> lateeoi irq acknowledgment") because the interrupt was disabled=20
> previously. Therefore we wouldn't do another iteration in=20
> 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.

> Aside the handlers, I think it may impact the defer EOI mitigation=20
> because in theory if a 3rd vCPU is joining the party (let say vCPU A=20
> migrate the event from vCPU B to vCPU C). So info->{eoi_cpu, irq_epoch,=
=20
> eoi_time} could possibly get mangled?
>=20
> For a fix, we may want to consider to hold evtchn_rwlock with the write=
=20
> permission. Although, I am not 100% sure this is going to prevent=20
> 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).

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().

> Does my write-up make sense to you?

Yes. What about my reply? ;-)


Juergen

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