Re: [PATCH 16/31] vhost: pass queue index to vhost_vq_get_addr

[Jason Wang]

在 2022/2/1 上午1:44, Eugenio Perez Martin 写道:
> On Sat, Jan 29, 2022 at 9:20 AM Jason Wang <jasowang@redhat.com> wrote:
> > 在 2022/1/22 上午4:27, Eugenio Pérez 写道:
> > > Doing that way allows vhost backend to know what address to return.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >    hw/virtio/vhost.c | 6 +++---
> > >    1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > > index 7b03efccec..64b955ba0c 100644
> > > --- a/hw/virtio/vhost.c
> > > +++ b/hw/virtio/vhost.c
> > > @@ -798,9 +798,10 @@ static int vhost_virtqueue_set_addr(struct vhost_dev *dev,
> > >                                        struct vhost_virtqueue *vq,
> > >                                        unsigned idx, bool enable_log)
> > >    {
> > > -    struct vhost_vring_addr addr;
> > > +    struct vhost_vring_addr addr = {
> > > +        .index = idx,
> > > +    };
> > >        int r;
> > > -    memset(&addr, 0, sizeof(struct vhost_vring_addr));
> > >
> > >        if (dev->vhost_ops->vhost_vq_get_addr) {
> > >            r = dev->vhost_ops->vhost_vq_get_addr(dev, &addr, vq);
> > > @@ -813,7 +814,6 @@ static int vhost_virtqueue_set_addr(struct vhost_dev *dev,
> > >            addr.avail_user_addr = (uint64_t)(unsigned long)vq->avail;
> > >            addr.used_user_addr = (uint64_t)(unsigned long)vq->used;
> > >        }
> >
> > I'm a bit lost in the logic above, any reason we need call
> > vhost_vq_get_addr() :) ?
> It's the way vhost_virtqueue_set_addr works if the backend has a
> vhost_vq_get_addr operation (currently, only vhost-vdpa). vhost first
> ask the address to the back end and then set it.


Right it's because vhost-vdpa doesn't use VA but GPA. But I'm not sure 
it's worth a dedicated vhost_ops. But consider we introduce shadow 
virtqueue stuffs, it should be ok now.

(In the future, we may consider to generalize non vhost-vdpa specific 
stuffs to VhostShadowVirtqueue, then we can get rid of this vhost_ops.


> Previously, index was not needed because all the information was in
> vhost_virtqueue. However to extract queue index from vhost_virtqueue
> is tricky, so I think it's easier to simply have that information at
> request, something similar to get_base or get_num when asking vdpa
> device. We can extract the index from vq - dev->vqs or something
> similar if it's prefered.


It looks odd for the caller to tell the index consider vhost_virtqueue 
is already passed. So I think we need deduce it from vhost_virtqueue as 
you mentioned here.

Thanks


> Thanks!
>
> > Thanks
> >
> >
> > > -    addr.index = idx;
> > >        addr.log_guest_addr = vq->used_phys;
> > >        addr.flags = enable_log ? (1 << VHOST_VRING_F_LOG) : 0;
> > >        r = dev->vhost_ops->vhost_set_vring_addr(dev, &addr);