There is no clearly defined purpose for the virtio status byte in
vhost-user: For resetting, we already have RESET_DEVICE; and for virtio
feature negotiation, we have [GS]ET_FEATURES. With the REPLY_ACK
protocol extension, it is possible for SET_FEATURES to return errors
(SET_PROTOCOL_FEATURES may be called before SET_FEATURES).
As for implementations, SET_STATUS is not widely implemented. dpdk does
implement it, but only uses it to signal feature negotiation failure.
While it does log reset requests (SET_STATUS 0) as such, it effectively
ignores them, in contrast to RESET_OWNER (which is deprecated, and today
means the same thing as RESET_DEVICE).
While qemu superficially has support for [GS]ET_STATUS, it does not
forward the guest-set status byte, but instead just makes it up
internally, and actually completely ignores what the back-end returns,
only using it as the template for a subsequent SET_STATUS to add single
bits to it. Notably, after setting FEATURES_OK, it never reads it back
to see whether the flag is still set, which is the only way in which
dpdk uses the status byte.
As-is, no front-end or back-end can rely on the other side handling this
field in a useful manner, and it also provides no practical use over
other mechanisms the vhost-user protocol has, which are more clearly
defined. Deprecate it.
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
docs/interop/vhost-user.rst | 28 +++++++++++++++++++++-------
1 file changed, 21 insertions(+), 7 deletions(-)