[PATCH v2] vdpa_sim: set last_used_idx as last_avail_idx in vdpasim_queue_ready

["Eugenio Pérez" <eperezma@redhat.com>]

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@redhat.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