Re: [PATCH v2 2/3] vhost: support PACKED when setting-getting vring_base

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 18, 2023 at 03:52:10PM +0800, Jason Wang wrote:
> On Thu, May 18, 2023 at 3:34 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
> >
> > On Thu, May 18, 2023 at 7:24 AM Jason Wang <jasowang@redhat.com> wrote:
> > >
> > > On Wed, May 17, 2023 at 3:00 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
> > > >
> > > > On Wed, May 17, 2023 at 7:26 AM Jason Wang <jasowang@redhat.com> wrote:
> > > > >
> > > > > On Wed, May 17, 2023 at 2:26 AM Shannon Nelson <shannon.nelson@amd.com> wrote:
> > > > > >
> > > > > > On 5/16/23 12:49 AM, Stefano Garzarella wrote:
> > > > > > > On Mon, May 15, 2023 at 01:41:12PM -0700, Shannon Nelson wrote:
> > > > > > >> On 5/9/23 1:46 AM, Stefano Garzarella wrote:
> > > > > > >>> On Mon, Apr 24, 2023 at 03:50:30PM -0700, Shannon Nelson via
> > > > > > >>> Virtualization wrote:
> > > > > > >>>> Use the right structs for PACKED or split vqs when setting and
> > > > > > >>>> getting the vring base.
> > > > > > >>>>
> > > > > > >>>> Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
> > > > > > >>>> ---
> > > > > > >>>> drivers/vhost/vhost.c | 18 +++++++++++++-----
> > > > > > >>>> drivers/vhost/vhost.h |  8 ++++++--
> > > > > > >>>> 2 files changed, 19 insertions(+), 7 deletions(-)
> > > > > > >>>>
> > > > > > >>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > > >>>> index f11bdbe4c2c5..f64efda48f21 100644
> > > > > > >>>> --- a/drivers/vhost/vhost.c
> > > > > > >>>> +++ b/drivers/vhost/vhost.c
> > > > > > >>>> @@ -1633,17 +1633,25 @@ long vhost_vring_ioctl(struct vhost_dev
> > > > > > >>>> *d, unsigned int ioctl, void __user *arg
> > > > > > >>>>                       r = -EFAULT;
> > > > > > >>>>                       break;
> > > > > > >>>>               }
> > > > > > >>>> -              if (s.num > 0xffff) {
> > > > > > >>>> -                      r = -EINVAL;
> > > > > > >>>> -                      break;
> > > > > > >>>> +              if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
> > > > > > >>>> +                      vq->last_avail_idx = s.num & 0xffff;
> > > > > > >>>> +                      vq->last_used_idx = (s.num >> 16) & 0xffff;
> > > > > > >>>> +              } else {
> > > > > > >>>> +                      if (s.num > 0xffff) {
> > > > > > >>>> +                              r = -EINVAL;
> > > > > > >>>> +                              break;
> > > > > > >>>> +                      }
> > > > > > >>>> +                      vq->last_avail_idx = s.num;
> > > > > > >>>>               }
> > > > > > >>>> -              vq->last_avail_idx = s.num;
> > > > > > >>>>               /* Forget the cached index value. */
> > > > > > >>>>               vq->avail_idx = vq->last_avail_idx;
> > > > > > >>>>               break;
> > > > > > >>>>       case VHOST_GET_VRING_BASE:
> > > > > > >>>>               s.index = idx;
> > > > > > >>>> -              s.num = vq->last_avail_idx;
> > > > > > >>>> +              if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> > > > > > >>>> +                      s.num = (u32)vq->last_avail_idx |
> > > > > > >>>> ((u32)vq->last_used_idx << 16);
> > > > > > >>>> +              else
> > > > > > >>>> +                      s.num = vq->last_avail_idx;
> > > > > > >>>
> > > > > > >>> The changes LGTM, but since we are changing the UAPI, should we
> > > > > > >>> update the documentation of VHOST_SET_VRING_BASE and
> > > > > > >>> VHOST_GET_VRING_BASE in include/uapi/linux/vhost.h?
> > > > > > >>
> > > > > > >> Correct me if I'm wrong, but I don't think we're changing anything in
> > > > > > >> the UAPI here, just fixing code to work correctly with what is already
> > > > > > >> happening.
> > > > > > >
> > > > > > > IIUC before this patch VHOST_GET_VRING_BASE and VHOST_SET_VRING_BASE
> > > > > > > never worked with packed virtqueue, since we were only handling
> > > > > > > last_avail_idx. Now we are supporting packed virtqueue, handling
> > > > > > > in vhost_vring_state.num both last_avail_idx and last_used_idx (with
> > > > > > > wrap counters).
> > > > > > >
> > > > > > > For example for VHOST_GET_VRING_BASE where is documented that the first
> > > > > > > 15 bits are last_avail_idx, the 16th the avail_wrap_counter, and the
> > > > > > > others are last_used_idx and used_wrap_counter?
> > > > > > >
> > > > > > > Maybe I missed something, but since this is UAPI, IMHO we should
> > > > > > > document the parameters of ioctls at least in
> > > > > > > include/uapi/linux/vhost.h.
> > > > > >
> > > > > > Perhaps Jason already has something written up that could be put in here
> > > > > > from when he first added the wrap_counter a couple of years ago?
> > > > >
> > > > > If you meant the virtio driver support for packed, I think it's
> > > > > different from the context which is vhost here.
> > > > >
> > > > > I agree with Stefano that we need to update the comments around
> > > > > GET_VRING_BASE and SET_VRING_BASE, then we are fine.
> > > >
> > > > I'm thinking if we should also add a new VHOST_BACKEND_F_RING_PACKED
> > > > feature (or something similar) to inform the user space that now we
> > > > are able to handle packed virtqueue through vhost IOCTLs, otherwise
> > > > how can the userspace know if it is supported or not?
> > >
> > > I probably understand this but I think it should be done via
> > > VHOST_GET_FEAETURES. It would be a burden if we matianing duplicated
> > > features.
> >
> > Good point, I see.
> >
> > I think we should do one of these things, though:
> > - mask VIRTIO_F_RING_PACKED in the stable kernels when
> > VHOST_GET_FEAETURES is called
> > - backport this patch on all stable kernels that support vhost-vdpa
> >
> > Maybe the last one makes more sense.
> 
> Not sure, it looks to me the packed support for vDPA was first added
> by Gautam. So it probably means that except for vp_vdpa, we don't have
> a vDPA parent that can do the packed virtuque now. Adding the relevant
> people here for more comment
> 
> Thanks

BTW should we fix up vhost.c to support packed rings too?
E.g. so we can migrate to vhost?
There's an old patchset of mine that started work on this:

https://lore.kernel.org/all/20200407011612.478226-1-mst%40redhat.com

Is there need for this now?

> >
> > Thanks,
> > Stefano
> >

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