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

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Message-ID: <20230228105628.lwd4l2nw6isfjc7c@sgarzare-redhat>
Date:   Tue, 28 Feb 2023 11:56:28 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Eugenio Pérez <eperezma@...hat.com>
Cc:     mst@...hat.com, 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,
        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
>---
> 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
>

If you need to resend, I'd add a comment in the code following the 
commit description.

Anyway, the patch LGTM:

Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

Thanks,
Stefano