Re: [PATCH 2/5] vdpa: Add vhost_vdpa_net_load_mq

[Jason Wang]

On Wed, Aug 24, 2022 at 3:47 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Wed, Aug 24, 2022 at 6:23 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > 在 2022/8/20 01:13, Eugenio Pérez 写道:
> > > Same way as with the MAC, restore the expected number of queues at
> > > device's start.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >   net/vhost-vdpa.c | 33 +++++++++++++++++++++++++++++++++
> > >   1 file changed, 33 insertions(+)
> > >
> > > diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
> > > index 1e0dbfcced..96fd3bc835 100644
> > > --- a/net/vhost-vdpa.c
> > > +++ b/net/vhost-vdpa.c
> > > @@ -391,6 +391,35 @@ static int vhost_vdpa_net_load_mac(VhostVDPAState *s,
> > >       return 0;
> > >   }
> > >
> > > +static int vhost_vdpa_net_load_mq(VhostVDPAState *s,
> > > +                                  const VirtIONet *n)
> > > +{
> > > +    uint64_t features = n->parent_obj.guest_features;
> > > +    ssize_t dev_written;
> > > +    void *cursor = s->cvq_cmd_out_buffer;
> > > +    if (!(features & BIT_ULL(VIRTIO_NET_F_MQ))) {
> > > +        return 0;
> > > +    }
> > > +
> > > +    *(struct virtio_net_ctrl_hdr *)cursor = (struct virtio_net_ctrl_hdr) 
> > > {
> > > +        .class = VIRTIO_NET_CTRL_MQ,
> > > +        .cmd = VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET,
> > > +    };
> > > +    cursor += sizeof(struct virtio_net_ctrl_hdr);
> > > +    *(struct virtio_net_ctrl_mq *)cursor = (struct virtio_net_ctrl_mq) {
> > > +        .virtqueue_pairs = cpu_to_le16(n->curr_queue_pairs),
> > > +    };
> >
> >
> > Such casting is not elegant, let's just prepare buffer and then do the
> > copy inside vhost_vdpa_net_cvq_add()?
> >
>
> I'm not sure what you propose here. I can pre-fill a buffer in the
> stack and then do an extra copy in vhost_vdpa_net_cvq_add. The
> compiler should be able to optimize it, but I'm not sure if it
> simplifies the code.
>
> We can have a dedicated buffer for mac, another for mq, and one for
> each different command, and map all of them at the device's start. But
> this seems too much overhead to me.

Considering we may need to support and restore a lot of other fields,
this looks a little complicated.

I meant the caller can simply do:

struct virtio_net_ctrl_mq mq = { ...};

Then we do

vhost_vdpa_net_cvq_add(&mq, sizeof(mq), ...);

Then we can do memcpy inside vhost_vdpa_net_cvq_add() and hide the
cmd_out_buffer etc from the caller.

>
> Some alternatives that come to my mind:
>
> * Declare a struct with both virtio_net_ctrl_hdr and each of the
> control commands (using unions?), and cast s->cvq_cmd_out_buffer
> accordingly.
> * Declare a struct with all of the supported commands one after
> another, and let qemu fill and send these accordingly.
>
> >
> > > +    cursor += sizeof(struct virtio_net_ctrl_mq);
> > > +
> > > +    dev_written = vhost_vdpa_net_cvq_add(s, cursor - 
> > > s->cvq_cmd_out_buffer,
> > > +                                             
> > > sizeof(virtio_net_ctrl_ack));
> > > +    if (unlikely(dev_written < 0)) {
> > > +        return dev_written;
> > > +    }
> > > +
> > > +    return *((virtio_net_ctrl_ack *)s->cvq_cmd_in_buffer) != 
> > > VIRTIO_NET_OK;
> >
> >
> > So I think we should have a dedicated buffer just for ack, then there's
> > no need for such casting.
> >
>
> You mean to declare cvq_cmd_in_buffer as virtio_net_ctrl_ack type
> directly and map it to the device?

Kind of, considering the ack is the only kind of structure in the near
future, can we simply use the structure virtio_net_ctl_ack?

Thanks

>
> Thanks!
>