Linux Virtualization - Oct 2023 - [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu affinity mask

On Mon, 23 Oct 2023 18:52:45 +0200, Jakub Sitnicki <jakub at
cloudflare.com> wrote:
> 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 at
cloudflare.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?

Actually, my question is that can we pass a val on the stack(or temp value) to
the irq_set_affinity_hint()?

Such as the virtio-net uses zalloc_cpumask_var to alloc a cpu_mask, and
that will be released.



	int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
				      bool setaffinity)
	{
		unsigned long flags;
		struct irq_desc *desc = irq_get_desc_lock(irq, &flags,
IRQ_GET_DESC_CHECK_GLOBAL);

		if (!desc)
			return -EINVAL;
->		desc->affinity_hint = m;
		irq_put_desc_unlock(desc, flags);
		if (m && setaffinity)
			__irq_set_affinity(irq, m, false);
		return 0;
	}
	EXPORT_SYMBOL_GPL(__irq_apply_affinity_hint);

The above code directly refers the mask pointer. If the mask is a temp value, I
think that is a bug.

And I notice that many places directly pass the temp value to this API.
And I am a little confused. ^_^ Or I missed something.

Thanks.

> >
> > 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

On Tue, Oct 24, 2023 at 10:31 AM +08, Xuan Zhuo wrote:
> On Mon, 23 Oct 2023 18:52:45 +0200, Jakub Sitnicki <jakub at
cloudflare.com> wrote:
>> 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 at
cloudflare.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?
>
>
> Actually, my question is that can we pass a val on the stack(or temp value)
to
> the irq_set_affinity_hint()?
>
> Such as the virtio-net uses zalloc_cpumask_var to alloc a cpu_mask, and
> that will be released.
>
>
>
> 	int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
> 				      bool setaffinity)
> 	{
> 		unsigned long flags;
> 		struct irq_desc *desc = irq_get_desc_lock(irq, &flags,
IRQ_GET_DESC_CHECK_GLOBAL);
>
> 		if (!desc)
> 			return -EINVAL;
> ->		desc->affinity_hint = m;
> 		irq_put_desc_unlock(desc, flags);
> 		if (m && setaffinity)
> 			__irq_set_affinity(irq, m, false);
> 		return 0;
> 	}
> 	EXPORT_SYMBOL_GPL(__irq_apply_affinity_hint);
>
> The above code directly refers the mask pointer. If the mask is a temp
value, I
> think that is a bug.
You are completely right. irq_set_affinity_hint stores the mask pointer.
irq_affinity_hint_proc_show later dereferences it when user reads out
/proc/irq/*/affinity_hint.

I have failed to notice that. That's why we need cpumask_copy to stay.

My patch is buggy. Please disregard.

I will send a v2 to only migrate from deprecated irq_set_affinity_hint.
> And I notice that many places directly pass the temp value to this API.
> And I am a little confused. ^_^ Or I missed something.
There seem two be two gropus of callers:

1. Those that use get_cpu_mask/cpumask_of/cpumask_of_node to produce a
   cpumask pointer which is a preallocated constant.

   $ weggli 'irq_set_affinity_hint(_, $func(_));' ~/src/linux

2. Those that pass a pointer to memory somewhere.

   $ weggli 'irq_set_affinity_hint(_, $mask);' ~/src/linux

(weggli tool can be found at https://github.com/weggli-rs/weggli)

I've looked over the callers from group #2 but I couldn't find any
passing a pointer memory on stack :-)

Thanks for pointing this out.

[...]