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

[Johannes Berg <johannes@sipsolutions.net>]

On Mon, 2019-09-09 at 11:45 -0400, Michael S. Tsirkin wrote:
> On Mon, Sep 09, 2019 at 05:34:13PM +0200, Johannes Berg wrote:
> > On Mon, 2019-09-09 at 17:26 +0200, Johannes Berg wrote:
> > > Maybe instead we should just add a "VHOST_USER_REPLY_ERROR" bit (e.g.
> > > bit 4 after NEED_REPLY). Qemu in vhost_user_read_header() validates that
> > > it received REPLY_MASK | VERSION, so it would reject the message at that
> > > point.
> > > 
> > > Another possibility would be to define the highest bit of the 'request'
> > > field to indicate an error, so for GET_FEATURES we'd return the value
> > > 0x80000000 | GET_FEATURES.
> > 
> > However, one way or another, that basically leaves us with three
> > different ways of indicating an error:
> > 
> >  1) already defined errors in existing messages - we can't change them
> >     since those are handled at runtime now, e.g. VHOST_USER_POSTCOPY_END
> >     returns a u64 value with an error status, and current code cannot
> >     deal with an error flag in the 'request' or 'flags' field
> >  2) F_REPLY_ACK errors to messages that do not specify a response at all
> >  3) this new way of indicating an error back from messages that specify
> >     a response, but the response has no inherent way of returning an
> >     error
> > 
> > To me that really feels a bit too complex from the spec POV. But I don't
> > see a way to generalize this without another extension, and again the
> > device cannot choose which extensions it supports since the master
> > chooses them and just sets them.
> > 
> > Perhaps I really should just stick a "g_assert()" into the code at that
> > point,
> 
> There's the old way: close the socket.
> This will make reads fail gracefully.
> If we don't want complexity right now, I'd go with that.

D'oh, good point. OK, I'll do that. Though it's almost equivalent in
libvhost-user to just asserting, since it's mostly set up to just handle
a single connection and then quit.

Alright, thanks. Like I said, I'll send some more patches all around
once I get it working, right now I'm crashing in some weird ways that I
need to debug :)

johannes