[PATCH v3] vdpa_sim: set last_used_idx as last_avail_idx in vdpasim_queue_ready

[Eugenio Pérez]

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 0e84f918fac8 ("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")
Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
Signed-off-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
---
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.

v3:
* Fix commit id in the patch message.
* Add in code comment following the patch message about why it is done
  this way.
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 | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
index 6a0a65814626..eea23c630f7c 100644
--- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
+++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
@@ -68,6 +68,17 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
 			  (uintptr_t)vq->device_addr);
 
 	vq->vring.last_avail_idx = last_avail_idx;
+
+	/*
+	 * 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.
+	 */
+	vq->vring.last_used_idx = last_avail_idx;
 	vq->vring.notify = vdpasim_vq_notify;
 }
 
-- 
2.31.1