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Date: Tue, 06 Apr 2010 09:36:17 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH 0/5] chardev, virtio-console: flow control,
	error handling
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To: Amit Shah <amit.shah@redhat.com>
Cc: qemu list <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>

   Hi,

> In case of EAGAIN, the unix_write() function just kept spinning while
> attempting to write to the chardev till it succeeded. This resulted in
> a stuck VM in case a chardev had opened connection but wasn't reading
> anything from qemu.

It spins only for non-blocking file handles.  In blocking mode the write 
syscall just goes sleep until it can write out data (or gets a signal).

> There are two fixes for that case:
> - Poll for POLLOUT instead of directly attempting write(), which helps
>    relax the CPU (and we become greener).

No need to do that, see above.  Having a poll() call in unix_write() is 
wrong IMHO.

> - If the file that we're writing to is nonblocking, return -EAGAIN to
>    the caller of qemu_chr_write() so that appropriate actions can be
>    taken higher up in the stack.

Good.

cheers,
   Gerd