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

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Date:   Tue, 20 Dec 2022 14:31:12 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.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 3:12 PM Yongji Xie <xieyongji@...edance.com> wrote:
>
> 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.

It's not necessarily the VM, do we have the same requirement for
userspace (like DPDK) drivers?

Thanks

>
> > >
> > > > 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
>