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Date:   Fri, 11 Dec 2020 11:13:31 +0100
From:   Thomas Gleixner <>
To:     Jürgen Groß <>,, 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 interrupts

On Fri, Dec 11 2020 at 07:17, Jürgen Groß wrote:
> 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.
> This binding to cpu0 was introduced with commit 97253eeeb792d61ed2
> and I have no reason to believe the underlying problem has been
> eliminated.

    "The kernel-side VCPU binding was not being correctly set for newly
     allocated or bound interdomain events.  In ARM guests where 2-level
     events were used, this would result in no interdomain events being
     handled because the kernel-side VCPU masks would all be clear.

     x86 guests would work because the irq affinity was set during irq
     setup and this would set the correct kernel-side VCPU binding."

I'm not convinced that this is really correctly analyzed because affinity
setting is done at irq startup.

                switch (__irq_startup_managed(desc, aff, force)) {
	        case IRQ_STARTUP_NORMAL:
	                ret = __irq_startup(desc);

which is completely architecture agnostic. So why should this magically
work on x86 and not on ARM if both are using the same XEN irqchip with
the same irqchip callbacks.



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