vdpa: fix RX failure after device reset by always using base 0

After a vDPA device reset, activate_vdpa() read avail_idx from guest
memory to pass as the vring base via VHOST_SET_VRING_BASE. However,
the guest memory still contained the stale avail_idx from the previous
session. For a 256-entry ring, this meant base=256, causing the
hardware to believe the entire RX ring was consumed with no available
buffers — RX silently stopped while TX continued to work.

QEMU handles this correctly by tracking last_avail_idx internally
(reset to 0 in virtio_reset()) and passing that value, rather than
reading from guest memory.

Fix by always passing base=0 to set_vring_base(). After a device
reset, both the guest driver and the vhost backend restart their rings
from index 0. For live migration, the correct base should come from
VHOST_GET_VRING_BASE (saved before the migration), not guest memory.

Tested with mlx5_vdpa (ConnectX-6 Dx) + Windows Server 2025 (netkvm).
Before: RX=0 after 3rd driver activation. After: full connectivity.

Fixes: cloud-hypervisor#7529
Signed-off-by: Max Makarov <maxpain@linux.com>

Author: maxpain

virtio-devices/src/vdpa.rs

@@ -266,16 +266,15 @@ impl Vdpa {
                 .unwrap()
                 .set_vring_addr(*queue_index, &config_data)
                 .map_err(Error::SetVringAddr)?;
+            // Always start from base 0. After device reset the guest
+            // driver restarts its rings from 0, but the old avail_idx
+            // value may still be present in guest memory (e.g. 256 for
+            // a 256-entry ring), which makes the hardware think the
+            // ring is full and no RX buffers are available.
             self.vhost
                 .as_ref()
                 .unwrap()
-                .set_vring_base(
-                    *queue_index,
-                    queue
-                        .avail_idx(mem, Ordering::Acquire)
-                        .map_err(Error::GetAvailableIndex)?
-                        .0,
-                )
+                .set_vring_base(*queue_index, 0)
                 .map_err(Error::SetVringBase)?;
 
             if let Some(eventfd) =