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 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, and have it crash, since it's likely that F_KICK_CALL_MSGS isn't
even going to be implemented in qemu (unless it grows simulation support
and then it'd all be conditional on some simulation command-line option)



And actually ... you got the order wrong:

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

That's not the case. We *start* with GET_FEATURES, if that includes
protocol features then we do GET_PROTOCOL_FEATURES next, and then we get
the # of queues next ...

Though the whole discussion pretty much applies equivalently to
GET_QUEUES_NUM instead of GET_FEATURES.

johannes