Re: [PATCH v4 03/11] virtio-vdpa: Support interrupt affinity spreading mechanism

[Yongji Xie]

On Fri, Mar 24, 2023 at 2:28 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> On Thu, Mar 23, 2023 at 1:31 PM Xie Yongji <xieyongji@xxxxxxxxxxxxx> wrote:
> >
> > To support interrupt affinity spreading mechanism,
> > this makes use of group_cpus_evenly() to create
> > an irq callback affinity mask for each virtqueue
> > of vdpa device. Then we will unify set_vq_affinity
> > callback to pass the affinity to the vdpa device driver.
> >
> > Signed-off-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
>
> Thinking hard of all the logics, I think I've found something interesting.
>
> Commit ad71473d9c437 ("virtio_blk: use virtio IRQ affinity") tries to
> pass irq_affinity to transport specific find_vqs().  This seems a
> layer violation since driver has no knowledge of
>
> 1) whether or not the callback is based on an IRQ
> 2) whether or not the device is a PCI or not (the details are hided by
> the transport driver)
> 3) how many vectors could be used by a device
>
> This means the driver can't actually pass a real affinity masks so the
> commit passes a zero irq affinity structure as a hint in fact, so the
> PCI layer can build a default affinity based that groups cpus evenly
> based on the number of MSI-X vectors (the core logic is the
> group_cpus_evenly). I think we should fix this by replacing the
> irq_affinity structure with
>
> 1) a boolean like auto_cb_spreading
>
> or
>
> 2) queue to cpu mapping
>

But only the driver knows which queues are used in the control path
which don't need the automatic irq affinity assignment. So I think the
irq_affinity structure can only be created by device drivers and
passed to the virtio-pci/virtio-vdpa driver.

> So each transport can do its own logic based on that. Then virtio-vDPA
> can pass that policy to VDUSE where we only need a group_cpus_evenly()
> and avoid duplicating irq_create_affinity_masks()?
>

I don't get why we would have duplicated irq_create_affinity_masks().

Thanks,
Yongji