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From:   Jakub Sitnicki <jakub@cloudflare.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        Jason Wang <jasowang@redhat.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        linux-kernel@vger.kernel.org, kernel-team@cloudflare.com
Subject: Re: [PATCH v2] virtio_pci: Switch away from deprecated
 irq_set_affinity_hint
Date:   Fri, 27 Oct 2023 12:03:17 +0200
In-reply-to: <20231026131830-mutt-send-email-mst@kernel.org>
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On Thu, Oct 26, 2023 at 01:20 PM -04, Michael S. Tsirkin wrote:
> 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@cloudflare.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.

Sorry, that is beyond my ken today.

I saw the comment in vp_modern_disable_vq_and_reset:

	/* For the case where vq has an exclusive irq, call synchronize_irq() to
	 * wait for completion.
	 *
	 * note: We can't use disable_irq() since it conflicts with the affinity
	 * managed IRQ that is used by some drivers.
	 */

... but I fail to follow how the two conflict.

Perhaps Xuah could shed some light here.