vdpa/sfc: resolve race between vhost lib and device conf

vdpa/sfc: resolve race between vhost lib and device conf

libvhost calls dev_conf() before prosessing the
VHOST_USER_SET_VRING_CALL message for the last VQ. So
this message is processed after dev_conf() returns.

However, the dev_conf() function spawns a thread to set
rte_vhost_host_notifier_ctrl() before returning control to
libvhost. This parallel thread in turn invokes get_notify_area().
To get the notify_area, the vdpa driver needs to query the HW and
for this query it needs an enabled VQ.

But at the same time libvhost is processing the last
VHOST_USER_SET_VRING_CALL, and to do that it disables the last VQ.

Hence there is a race b/w the libvhost and the vdpa driver.

To resolve this race condition, query the HW and cache notify_area
inside dev_conf() instead of doing it the parallel thread.

Fixes: 630be406dcbf ("vdpa/sfc: get queue notify area info")
Cc: stable@dpdk.org
Signed-off-by: Abhimanyu Saini <absaini@amd.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>