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

[Daniel P. Berrange]

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.

Regards,
Daniel
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