Re: [Virtio-fs] (no subject)

[Hanna Czenczek <hreitz@redhat.com>]

On 05.10.23 19:15, Michael S. Tsirkin wrote:
> On Thu, Oct 05, 2023 at 01:08:52PM -0400, Stefan Hajnoczi wrote:
>> 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>
>
> SET_STATUS is the only way to signal failure to acknowledge FEATURES_OK.
> The fact current backends never check errors does not mean they never
> will. So no, not applying this.

Can this not be done with REPLY_ACK?  I.e., with the following message 
order:

1. GET_FEATURES to find out whether VHOST_USER_F_PROTOCOL_FEATURES is 
present
2. GET_PROTOCOL_FEATURES to hopefully get VHOST_USER_PROTOCOL_F_REPLY_ACK
3. SET_PROTOCOL_FEATURES to set VHOST_USER_PROTOCOL_F_REPLY_ACK
4. SET_FEATURES with need_reply

If not, the problem is that qemu has sent SET_STATUS 0 for a while when 
the vCPUs are stopped, which generally seems to request a device reset.  
If we don’t state at least that SET_STATUS 0 is to be ignored, back-ends 
that will implement SET_STATUS later may break with at least these qemu 
versions.  But documenting that a particular use of the status byte is 
to be ignored would be really strange.

Hanna