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 18:14:04 +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 Tue, Dec 20, 2022 at 2:31 PM Jason Wang <jasowang@...hat.com> wrote:
>
> 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?
>

IIUC the vhost-vdpa's irq callback just signals the eventfd. I didn't
see how to use the irq affinity hint in vdpa device driver. The real
irq callback should be called in DPDK internally.

Thanks,
Yongji