Re: [PATCH v4 1/8] vhost-user.rst: Deprecate [GS]ET_STATUS

[Stefan Hajnoczi]

On Wed, Oct 04, 2023 at 02:58:57PM +0200, Hanna Czenczek wrote:
> 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(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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