From: "Michael S. Tsirkin" <mst@redhat.com>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [RFC] libvhost-user: implement VHOST_USER_PROTOCOL_F_KICK_CALL_MSGS
Date: Mon, 9 Sep 2019 11:45:28 -0400 [thread overview]
Message-ID: <20190909114358-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <f97477a5b16e69a4891c8da542f5002fe4cf2548.camel@sipsolutions.net>
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.
> 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
next prev parent reply other threads:[~2019-09-09 15:46 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-02 12:12 [Qemu-devel] [RFC] vhost-user simulation extension Johannes Berg
2019-09-02 12:12 ` [Qemu-devel] [RFC] docs: vhost-user: add in-band kick/call messages Johannes Berg
2019-09-05 20:28 ` Johannes Berg
2019-09-09 16:00 ` Stefan Hajnoczi
2019-09-09 17:34 ` Johannes Berg
2019-09-10 15:03 ` Stefan Hajnoczi
2019-09-10 15:14 ` Johannes Berg
2019-09-10 15:33 ` Michael S. Tsirkin
2019-09-10 15:34 ` Johannes Berg
2019-09-11 6:56 ` Stefan Hajnoczi
2019-09-11 7:35 ` Stefan Hajnoczi
2019-09-11 8:26 ` Johannes Berg
2019-09-11 15:17 ` Stefan Hajnoczi
2019-09-11 15:31 ` Stefan Hajnoczi
2019-09-11 15:36 ` Johannes Berg
2019-09-11 15:38 ` Johannes Berg
2019-09-12 12:22 ` Stefan Hajnoczi
2019-09-12 20:37 ` Johannes Berg
2019-09-06 12:13 ` [Qemu-devel] [RFC] libvhost-user: implement VHOST_USER_PROTOCOL_F_KICK_CALL_MSGS Johannes Berg
2019-09-06 14:22 ` Michael S. Tsirkin
2019-09-06 14:48 ` Johannes Berg
2019-09-06 15:12 ` Michael S. Tsirkin
2019-09-06 15:32 ` Johannes Berg
2019-09-08 13:13 ` Michael S. Tsirkin
2019-09-09 11:35 ` Johannes Berg
2019-09-09 12:41 ` Michael S. Tsirkin
2019-09-09 13:05 ` Johannes Berg
2019-09-09 13:48 ` Michael S. Tsirkin
2019-09-09 13:50 ` Johannes Berg
2019-09-09 14:59 ` Michael S. Tsirkin
2019-09-09 15:26 ` Johannes Berg
2019-09-09 15:34 ` Johannes Berg
2019-09-09 15:45 ` Michael S. Tsirkin [this message]
2019-09-09 15:47 ` Johannes Berg
2019-09-10 15:52 ` Johannes Berg
2019-09-11 9:16 ` Michael S. Tsirkin
2019-09-11 9:20 ` Johannes Berg
2019-09-11 9:54 ` Michael S. Tsirkin
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