linux-kernel - Re: [PATCH v2] vdpa_sim: set last_used_idx as last_avail_idx in vdpasim_queue

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Date:   Thu, 2 Mar 2023 19:19:55 +0100
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     lulu@...hat.com, virtualization@...ts.linux-foundation.org,
        si-wei.liu@...cle.com, leiyang@...hat.com,
        Gautam Dawar <gdawar@...inx.com>, Eli Cohen <elic@...dia.com>,
        longpeng2@...wei.com, parav@...dia.com,
        linux-kernel@...r.kernel.org, sgarzare@...hat.com,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        alvaro.karsz@...id-run.com, Laurent Vivier <lvivier@...hat.com>
Subject: Re: [PATCH v2] vdpa_sim: set last_used_idx as last_avail_idx in vdpasim_queue_ready

On Tue, Feb 28, 2023 at 1:13 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Feb 03, 2023 at 03:25:01PM +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.
> >
> > Since vdpa_sim does not support receive inflight descriptors as a
> > destination of a migration, let's set both avail_idx and used_idx the
> > same at vq start.  This is how vhost-user works in a
> > VHOST_SET_VRING_BASE call.
> >
> > Although the simple fix is to set last_used_idx at vdpasim_set_vq_state,
> > it would be reset at vdpasim_queue_ready.  The last_avail_idx case is
> > fixed with commit a09f493c ("vdpa_sim: not reset state in
> > vdpasim_queue_ready").  Since the only option is to make it equal to
> > last_avail_idx, adding the only change needed here.
> >
> > This was discovered and tested live migrating the vdpa_sim_net device.
> >
> > Fixes: 2c53d0f64c06 ("vdpasim: vDPA device simulator")
> > Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> > ---
> > Cherry-picked from patch 2/2 of the series [1]. Differences are:
> > * Set the value of used_idx at vdpasim_queue_ready instead of fetching
> >   from the guest vring like vhost-kernel.
> >
> > v2: Actually update last_used_idx only at vdpasim_queue_ready.
> >
> > Note that commit id present in the patch text is not in master but in
> > git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git.
> >
> > [1] https://lkml.org/lkml/2023/1/18/1041
>
>
> Can you post with a fixed hash please?
>

Sent v3, please let me know if I need to do further modifications.

Thanks!

> > ---
> >  drivers/vdpa/vdpa_sim/vdpa_sim.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > index 6a0a65814626..79ac585e40b9 100644
> > --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> > @@ -68,6 +68,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
> >                         (uintptr_t)vq->device_addr);
> >
> >       vq->vring.last_avail_idx = last_avail_idx;
> > +     vq->vring.last_used_idx = last_avail_idx;
> >       vq->vring.notify = vdpasim_vq_notify;
> >  }
> >
> > --
> > 2.31.1
>