linux-kernel - Re: [PATCH v2 03/11] vdpa: Add set_irq_affinity callback in vdpa_config

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Date:   Mon, 19 Dec 2022 15:12:07 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 03/11] vdpa: Add set_irq_affinity callback in vdpa_config_ops

On Mon, Dec 19, 2022 at 2:06 PM Jason Wang <jasowang@...hat.com> wrote:
>
> On Mon, Dec 19, 2022 at 12:39 PM Yongji Xie <xieyongji@...edance.com> wrote:
> >
> > On Fri, Dec 16, 2022 at 11:58 AM Jason Wang <jasowang@...hat.com> wrote:
> > >
> > > On Mon, Dec 5, 2022 at 4:43 PM Xie Yongji <xieyongji@...edance.com> wrote:
> > > >
> > > > This introduces set_irq_affinity callback in
> > > > vdpa_config_ops so that vdpa device driver can
> > > > get the interrupt affinity hint from the virtio
> > > > device driver. The interrupt affinity hint would
> > > > be needed by the interrupt affinity spreading
> > > > mechanism.
> > > >
> > > > Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> > > > ---
> > > >  drivers/virtio/virtio_vdpa.c | 4 ++++
> > > >  include/linux/vdpa.h         | 8 ++++++++
> > > >  2 files changed, 12 insertions(+)
> > > >
> > > > diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
> > > > index 08084b49e5a1..4731e4616ee0 100644
> > > > --- a/drivers/virtio/virtio_vdpa.c
> > > > +++ b/drivers/virtio/virtio_vdpa.c
> > > > @@ -275,9 +275,13 @@ static int virtio_vdpa_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
> > > >         struct virtio_vdpa_device *vd_dev = to_virtio_vdpa_device(vdev);
> > > >         struct vdpa_device *vdpa = vd_get_vdpa(vdev);
> > > >         const struct vdpa_config_ops *ops = vdpa->config;
> > > > +       struct irq_affinity default_affd = { 0 };
> > > >         struct vdpa_callback cb;
> > > >         int i, err, queue_idx = 0;
> > > >
> > > > +       if (ops->set_irq_affinity)
> > > > +               ops->set_irq_affinity(vdpa, desc ? desc : &default_affd);
> > >
> > > I wonder if we need to do this in vhost-vDPA.
> >
> > I don't get why we need to do this in vhost-vDPA? Should this be done in VM?
>
> If I was not wrong, this tries to set affinity on the host instead of
> the guest. More below.
>

Yes, it's host stuff. This is used by the virtio device driver to pass
the irq affinity hint (tell which irq vectors don't need affinity
management) to the irq affinity manager. In the VM case, it should
only be related to the guest's virtio device driver and pci irq
affinity manager. So I don't get why we need to do this in vhost-vDPA.

> >
> > > Or it's better to have a
> > > default affinity by the vDPA parent
> > >
> >
> > I think both are OK. But the default value should always be zero, so I
> > put it in a common place.
>
> I think we should either:
>
> 1) document the zero default value in vdpa.c
> 2) set the zero in both vhost-vdpa and virtio-vdpa, or in the vdpa core
>

Can we only call it in the virtio-vdpa case? Thus the vdpa device
driver can know whether it needs to do the automatic irq affinity
management or not. In the vhost-vdpa case, we actually don't need the
irq affinity management.

Thanks,
Yongji