Re: [Qemu-devel] [PATCH RFC 1/6] io: only allow return path for socket typed

[Peter Xu]

On Fri, May 19, 2017 at 09:30:10AM +0100, Daniel P. Berrange wrote:
> On Fri, May 19, 2017 at 09:25:38AM +0100, Daniel P. Berrange wrote:
> > On Fri, May 19, 2017 at 02:43:27PM +0800, Peter Xu wrote:
> > > We don't really have a return path for the other types yet. Let's check
> > > this when .get_return_path() is called.
> > > 
> > > For this, we introduce a new feature bit, and set it up only for socket
> > > typed IO channels.
> > > 
> > > This will help detect earlier failure for postcopy, e.g., logically
> > > speaking postcopy cannot work with "exec:". Before this patch, when we
> > > try to migrate with "migrate -d exec:cat>out", we'll hang the system.
> > > With this patch, we'll get:
> > > 
> > > (qemu) migrate -d exec:cat>out
> > > Unable to open return-path for postcopy
> > 
> > This is wrong - post-copy migration *can* work with exec: - it just entirely
> > depends on what command you are running. Your example ran a command which is
> > unidirectional, but if you ran 'exec:socat ...' you would have a fully
> > bidirectional channel. Actually the channel is always bi-directional, but
> > 'cat' simply won't ever send data back to QEMU.
> > 
> > If QEMU hangs when the other end doesn't send data back, that actually seems
> > like a potentially serious bug in migration code. Even if using the normal
> > 'tcp' migration protocol, if the target QEMU server hangs and fails to
> > send data to QEMU on the return path, the source QEMU must never hang.
> 
> BTW, if you want to simplify the code in this area at all, then arguably
> we should get rid of the "get_return_path" helper method entirely. We're
> not actually opening any new connections - we're just creating a second
> QEMUFile that uses the same underlying QIOChannel object. All we would
> need is for the QEMUFile to have a separate 'buf' field management in
> QEMUFile for the read & write directions.  Then all the code would be
> able to just use the single QEMUFile for read & write getting rid of this
> concept of "opening a return path" which doens't actually do anything at
> the underlying data transport level.

Makes sense. Noted. Thanks,

-- 
Peter Xu