linux-kernel - Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init

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Message-ID: <CAJaqyWdaTY=fHpKxOhWw1Ap-FT4taZB3FogVuMa-Ackjdfz0vw@mail.gmail.com>
Date:   Wed, 1 Feb 2023 18:24:06 +0100
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     leiyang@...hat.com, Laurent Vivier <lvivier@...hat.com>,
        sgarzare@...hat.com, jasowang@...hat.com,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        virtualization@...ts.linux-foundation.org, si-wei.liu@...cle.com,
        linux-kernel@...r.kernel.org, lulu@...hat.com,
        Gautam Dawar <gdawar@...inx.com>, alvaro.karsz@...id-run.com
Subject: Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

On Wed, Feb 1, 2023 at 5:11 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Wed, Jan 18, 2023 at 05:43:59PM +0100, Eugenio Pérez 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@...hat.com>
>
>
> So I applied 1/2 and dropped 2/2 for now, right?
>

Yes, please. 2/2 needs tweaking, I'll address them ASAP.

Thanks!

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