On 18.07.23 17:10, Stefan Hajnoczi wrote:
On Tue, Jul 11, 2023 at 05:52:28PM +0200, Hanna Czenczek wrote:
The only user of vhost_user_reset_status() is vhost_dev_stop(), which
only uses it as a fall-back to stop the back-end if it does not support
SUSPEND. However, vhost-user's implementation is a no-op unless the
back-end supports SET_STATUS.
vhost-vdpa's implementation instead just calls
vhost_vdpa_reset_device(), implying that it's OK to fully reset the
device if SET_STATUS is not supported.
To be fair, vhost_vdpa_reset_device() does nothing but to set the status
to zero. However, that may well be because vhost-vdpa has no method
besides this to reset a device. In contrast, vhost-user has
RESET_DEVICE and a RESET_OWNER, which can be used instead.
While it is not entirely clear from documentation or git logs, from
discussions and the order of vhost-user protocol features, it appears to
me as if RESET_OWNER originally had no real meaning for vhost-user, and
was thus used to signal a device reset to the back-end. Then,
RESET_DEVICE was introduced, to have a well-defined dedicated reset
command. Finally, vhost-user received full STATUS support, including
SET_STATUS, so setting the device status to 0 is now the preferred way
of resetting a device. Still, RESET_DEVICE and RESET_OWNER should
remain valid as fall-backs.
Therefore, have vhost_user_reset_status() fall back to
vhost_user_reset_device() if the back-end has no STATUS support.
Signed-off-by: Hanna Czenczek <hre...@redhat.com>
---
hw/virtio/vhost-user.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 4507de5a92..53a881ec2a 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -2833,6 +2833,8 @@ static void vhost_user_reset_status(struct vhost_dev *dev)
if (virtio_has_feature(dev->protocol_features,
VHOST_USER_PROTOCOL_F_STATUS)) {
vhost_user_set_status(dev, 0);
+ } else {
+ vhost_user_reset_device(dev);
}
}
Did you check whether DPDK treats setting the status to 0 as equivalent
to RESET_DEVICE?
If it doesn’t, what’s even the point of using reset_status?
I will investigate, but if there’s a difference, that makes the whole
reset_* thing even more questionable to me than it has already been so far.
Hanna
My understanding is that SET_STATUS is mostly ignored by vhost-user
back-ends today. Even those that implement it may not treat SET_STATUS 0
as equivalent to RESET_DEVICE.
If you decide it's safe to make this change, please also update
vhost-user.rst to document that front-ends should use SET_STATUS 0,
RESET_DEVICE, and RESET_OWNER in order of preference.
Stefan
