linux-kernel - Re: [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu affinity mask

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Message-ID: <87il6x2rj6.fsf@cloudflare.com>
Date:   Mon, 23 Oct 2023 18:52:45 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-team@...udflare.com, Caleb Raitto <caraitto@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu
 affinity mask

On Thu, Oct 19, 2023 at 08:55 PM +08, Xuan Zhuo wrote:
> On Thu, 19 Oct 2023 12:16:24 +0200, Jakub Sitnicki <jakub@...udflare.com> wrote:
>> Since commit 19e226e8cc5d ("virtio: Make vp_set_vq_affinity() take a
>> mask.") it is actually not needed to have a local copy of the cpu mask.
>
>
> Could you give more info to prove this?
>
> If you are right, I think you should delete all code about msix_affinity_masks?

Sorry for the late reply. I've been away.

It looks that msix_affinity_masks became unused - intentionally - in
2015, after commit 210d150e1f5d ("virtio_pci: Clear stale cpumask when
setting irq affinity") [1].

Originally introduced in 2012 in commit 75a0a52be3c2 ("virtio: introduce
an API to set affinity for a virtqueue") [2]. As I understand, it was
meant to make it possible to set VQ affinity to more than once CPU.

Now that we can pass a CPU mask, listing all CPUs, to set_vq_affinity,
msix_affinity_masks seems to no longer have a purpose.

So, IMO, you're right. We can remove it.

Happy to do that in a follow up series.

That is - if you're okay with these two patches in the current form.

Thanks for reviewing.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=210d150e1f5da506875e376422ba31ead2d49621
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a0a52be3c27b58654fbed2c8f2ff401482b9a4