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Date: Mon, 12 Apr 2010 09:51:51 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH 0/8] (v2) chardev,
 virtio-console: flow control, error handling, fixes
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To: Paul Brook <paul@codesourcery.com>
Cc: Amit Shah <amit.shah@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, qemu list <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>

On 04/08/10 00:58, Paul Brook wrote:
>> Hello,
>>
>> This patchset introduces flow control to virtio-console and
>> chardev-based virtio serial ports. This series is based on the
>> previous series I sent on Mar 31st (00/17: v4: virtio-serial fixes,
>> new abi for port discovery)
>>
>> The qemu chardevs can now return -EAGAIN when a non-blocking remote
>> isn't ready to accept more data.
>>
>> Comments?
>
> This is a major change in semantics. Are you sure all users handle this
> correctly? My guess is that most of the devices don't.

I don't expect trouble here.  EAGAIN is returned only for file handles 
in non-blocking mode.  I doubt existing users use non-blocking I/O as 
this makes the current unix_write() code go busy-loop in case the 
outgoing pipe is full.

> EAGAIN isn't really a useful response unless you have some way of notifying
> the device that it can send more data.

This is a valid point though.

cheers,
   Gerd