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Date:   Fri, 11 Dec 2020 13:10:43 +0100
From:   Jürgen Groß <>
To:, Thomas Gleixner <>,
        LKML <>
Cc:     Peter Zijlstra <>,
        Marc Zyngier <>,
        Stefano Stabellini <>,,
        "James E.J. Bottomley" <>,
        Helge Deller <>,
        afzal mohammed <>,, Russell King <>,,
        Mark Rutland <>,
        Catalin Marinas <>,
        Will Deacon <>,
        Christian Borntraeger <>,
        Heiko Carstens <>,,
        Jani Nikula <>,
        Joonas Lahtinen <>,
        Rodrigo Vivi <>,
        David Airlie <>,
        Daniel Vetter <>,
        Pankaj Bharadiya <>,
        Chris Wilson <>,
        Wambui Karuga <>,,,
        Tvrtko Ursulin <>,
        Linus Walleij <>,, Lee Jones <>,
        Jon Mason <>,
        Dave Jiang <>,
        Allen Hubbe <>,,
        Lorenzo Pieralisi <>,
        Rob Herring <>,
        Bjorn Helgaas <>,
        Michal Simek <>,,
        Karthikeyan Mitran <>,
        Hou Zhiqiang <>,
        Tariq Toukan <>,
        "David S. Miller" <>,
        Jakub Kicinski <>,,, Saeed Mahameed <>,
        Leon Romanovsky <>
Subject: Re: [patch 27/30] xen/events: Only force affinity mask for percpu

On 11.12.20 00:20, 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.

After some discussion with Thomas on IRC and xen-devel archaeology the
result is: this will be needed especially for systems running on a
single vcpu (e.g. small guests), as the .irq_set_affinity() callback
won't be called in this case when starting the irq.


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