Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Jason Wang]

On Thu, Jan 19, 2023 at 4:11 PM Eugenio Perez Martin
<eperezma@xxxxxxxxxx> wrote:
>
> On Thu, Jan 19, 2023 at 4:20 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >
> > On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@xxxxxxxxxx> wrote:
> > >
> > > Starting from an used_idx different than 0 is needed in use cases like
> > > virtual machine migration.  Not doing so and letting the caller set an
> > > avail idx different than 0 causes destination device to try to use old
> > > buffers that source driver already recover and are not available
> > > anymore.
> > >
> > > While callers like vdpa_sim set avail_idx directly it does not set
> > > used_idx.  Instead of let the caller do the assignment, fetch it from
> > > the guest at initialization like vhost-kernel do.
> > >
> > > To perform the same at vring_kernel_init and vring_user_init is left for
> > > the future.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
> > > ---
> > >  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
> > >  1 file changed, 23 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > index 33eb941fcf15..0eed825197f2 100644
> > > --- a/drivers/vhost/vringh.c
> > > +++ b/drivers/vhost/vringh.c
> > > @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
> > >         return 0;
> > >  }
> > >
> > > +/**
> > > + * vringh_update_used_idx - fetch used idx from driver's used split vring
> > > + * @vrh: The vring.
> > > + *
> > > + * Returns -errno or 0.
> > > + */
> > > +static inline int vringh_update_used_idx(struct vringh *vrh)
> > > +{
> > > +       return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> > > +}
> > > +
> > >  /**
> > >   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
> > >   * @vrh: the vringh to initialize.
> > > @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
> > >                       struct vring_avail *avail,
> > >                       struct vring_used *used)
> > >  {
> >
> > While at this, I wonder if it's better to have a dedicated parameter
> > for last_avail_idx?
> >
>
> I also had that thought. To directly assign last_avail_idx is not a
> specially elegant API IMO.
>
> Maybe expose a way to fetch used_idx from device vring and pass
> used_idx as parameter too?

If I was not wrong, we can start from last_avail_idx, for used_idx it
is only needed for inflight descriptors which might require other
APIs?

(All the current vDPA user of vringh is doing in order processing)

>
> > > -       return vringh_init_kern(vrh, features, num, weak_barriers,
> > > -                               desc, avail, used);
> > > +       int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> > > +                                avail, used);
> > > +
> > > +       if (r != 0)
> > > +               return r;
> > > +
> > > +       /* Consider the ring not initialized */
> > > +       if ((void *)desc == used)
> > > +               return 0;
> >
> > I don't understand when we can get this (actually it should be a bug
> > of the caller).
> >
>
> You can see it in vdpasim_vq_reset.
>
> Note that to consider desc == 0 to be an uninitialized ring is a bug
> IMO. QEMU considers it that way also, but the standard does not forbid
> any ring to be at address 0. Especially if we use vIOMMU.
>
> So I think the best way to know if we can use the vringh is either
> this way, or provide an explicit "initialized" boolean attribute.
> Maybe a new "bool is_initialized(vrh)" is enough, if we don't want to
> add new attributes.

I wonder if we can avoid this in the simulator level instead of the
vringh (anyhow it only exposes a vringh_init_xxx() helper now).

Thanks

>
> Thanks!
>
> > Thanks
> >
> > > +
> > > +       return vringh_update_used_idx(vrh);
> > > +
> > >  }
> > >  EXPORT_SYMBOL(vringh_init_iotlb);
> > >
> > > --
> > > 2.31.1
> > >
> >
>