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

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Date:   Tue, 28 Feb 2023 07:13:00 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eugenio Pérez <eperezma@...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 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?

> ---
>  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