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From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
Message-ID: <1831964f-185e-31bb-2446-778f2c18d71b@suse.com>
Date:   Mon, 8 Feb 2021 13:14:13 +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,
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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: <1831964f-185e-31bb-2446-778f2c18d71b@suse.com>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
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On 08.02.21 11:40, Julien Grall wrote:
> Hi Juergen,
>=20
> On 08/02/2021 10:22, J=C3=BCrgen Gro=C3=9F wrote:
>> On 08.02.21 10:54, Julien Grall wrote:
>>> ... I don't really see how the difference matter here. The idea is to=
=20
>>> re-use what's already existing rather than trying to re-invent the=20
>>> wheel with an extra lock (or whatever we can come up).
>>
>> The difference is that the race is occurring _before_ any IRQ is
>> involved. So I don't see how modification of IRQ handling would help.
>=20
> Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:
>=20
> if ( irq in progress )
> {
>  =C2=A0 set IRQS_PENDING
>  =C2=A0 return;
> }
>=20
> do
> {
>  =C2=A0 clear IRQS_PENDING
>  =C2=A0 handle_irq()
> } while (IRQS_PENDING is set)
>=20
> IRQ handling flow like handle_fasteoi_irq() looks like:
>=20
> if ( irq in progress )
>  =C2=A0 return;
>=20
> handle_irq()
>=20
> The latter flow would catch "spurious" interrupt and ignore them. So it=
=20
> would handle nicely the race when changing the event affinity.

Sure? Isn't "irq in progress" being reset way before our "lateeoi" is
issued, thus having the same problem again? And I think we want to keep
the lateeoi behavior in order to be able to control event storms.


Juergen

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