netdev - Re: [patch 27/30] xen/events: Only force affinity mask for percpu interrupts

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Message-ID: <87ft4di4t8.fsf@nanos.tec.linutronix.de>
Date:   Fri, 11 Dec 2020 01:06:59 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     boris.ostrovsky@...cle.com, LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Marc Zyngier <maz@...nel.org>, Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        afzal mohammed <afzal.mohd.ma@...il.com>,
        linux-parisc@...r.kernel.org, Russell King <linux@...linux.org.uk>,
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        Will Deacon <will@...nel.org>,
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        Heiko Carstens <hca@...ux.ibm.com>, linux-s390@...r.kernel.org,
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        linux-rdma@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [patch 27/30] xen/events: Only force affinity mask for percpu interrupts

On Thu, Dec 10 2020 at 18:20, boris ostrovsky wrote:
> On 12/10/20 2:26 PM, Thomas Gleixner wrote:
>> All event channel setups bind the interrupt on CPU0 or the target CPU for
>> percpu interrupts and overwrite the affinity mask with the corresponding
>> cpumask. That does not make sense.
>>
>> The XEN implementation of irqchip::irq_set_affinity() already picks a
>> single target CPU out of the affinity mask and the actual target is stored
>> in the effective CPU mask, so destroying the user chosen affinity mask
>> which might contain more than one CPU is wrong.
>>
>> Change the implementation so that the channel is bound to CPU0 at the XEN
>> level and leave the affinity mask alone. At startup of the interrupt
>> affinity will be assigned out of the affinity mask and the XEN binding will
>> be updated. 
>
> If that's the case then I wonder whether we need this call at all and
> instead bind at startup time.

I was wondering about that, but my knowledge about the Xen internal
requirements is pretty limited. The current set at least survived basic
testing by Jürgen.

Thanks,

        tglx