Re: [Qemu-devel] [RFC] libvhost-user: implement VHOST_USER_PROTOCOL_F_KICK_CALL_MSGS

[Michael S. Tsirkin]

On Mon, Sep 09, 2019 at 03:05:08PM +0200, Johannes Berg wrote:
> On Mon, 2019-09-09 at 08:41 -0400, Michael S. Tsirkin wrote:
> 
> > > The only thing we could do is crash if it wants to use this feature
> > > without the others, but would that really be helpful?
> > 
> > We can return failure from SET_PROTOCOL_FEATURES.
> > We can also fail all following messages.
> 
> Only if we have F_REPLY_ACK, I think?
> 
> I suppose we could fail a later command that already needs a reply
> without REPLY_ACK, but that seems difficult to debug?


Next command is GET_FEATURES. Return an error response from that
and device init will fail.

> Anyway, if you feel that we should have this as some sort of safeguard I
> can try to do that; to me it feels rather pointless as libvhost-user is
> more of a sample implementation than anything else.


Exactly for this reason :)

> Unless you also wanted to write into the spec that F_KICK_CALL_MSGS
> absolutely *requires* F_REPLY_ACK,

yep

> and add some text that tells a device
> how to behave when F_KICK_CALL_MSGS is negotiated without F_REPLY_ACK?
> 
> johannes

We can document how to behave in case of inconsistent protocol features,
yes.