linux-kernel - Re: [PATCH v2] virtio_pci: Switch away from deprecated irq_set_affinity

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Date:   Thu, 26 Oct 2023 13:20:34 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     virtualization@...ts.linux-foundation.org,
        Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [PATCH v2] virtio_pci: Switch away from deprecated
 irq_set_affinity_hint

On Thu, Oct 26, 2023 at 06:25:08PM +0200, Jakub Sitnicki wrote:
> On Wed, Oct 25, 2023 at 04:53 PM +02, Jakub Sitnicki wrote:
> > Since commit 65c7cdedeb30 ("genirq: Provide new interfaces for affinity
> > hints") irq_set_affinity_hint is being phased out.
> >
> > Switch to new interfaces for setting and applying irq affinity hints.
> >
> > Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> > ---
> > v2:
> >  - Leave cpumask_copy as is. We can't pass pointer to stack memory as hint.
> >    Proposed a change to IRQ affinity interface to address this limitation:
> >    https://lore.kernel.org/r/20231025141517.375378-1-jakub@cloudflare.com
> 
> Just a note to the ^ - if we wanted to get rid of msix_affinity_masks,
> we could call irq_set_affinity directly, instead of calling it through
> irq_set_affinity[_and]_hint.
> 
> The hint wouldn't be available any more in /proc/irq/N/affinity_hint,
> but the same information can be gathered from /proc/irq/N/smp_affinity.
> 
> [...]


So we are potentially breaking some userspace - what's the value we
gain?  Is there some way we can make disable_irq/enable_irq work?
That would have a lot of value.
There is an actual need for that in virtio for coco but we can't use
these APIs with affinity managed IRQs.

-- 
MST