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Date: Tue, 08 Feb 2011 11:58:04 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: [Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by
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To: Aurelien Jarno <aurelien@aurel32.net>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Anthony Liguori <aliguori@linux.vnet.ibm.com>, Paul Brook <paul@codesourcery.com>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Arun Bharadwaj <arun@linux.vnet.ibm.com>

On 02/08/2011 04:06 AM, Aurelien Jarno wrote:
> Yes, it's slow. But is it a problem? You assume that people use QEMU
> only for emulating SMP platforms. This is a wrong assumption. Beside the
> x86 target, only sparc really supports SMP emulation.
>    

It's *not* just about performance.

TCG requires a signal to break out of a tight chained TB loop.  If you 
have a guest in a tight loop waiting for something external (like 
polling on a in-memory flag), the device emulation will not get to run 
until a signal is fired.

Unless you set SIGIO on every file descriptor that selects polls on (and 
you can't because there are a number that just don't support SIGIO), 
then you have a race condition.

This can be fixed by running TCG in a separate thread than select() and 
sending a signal to the TCG VCPU when select() returns (effectively 
SIGIO in userspace).

This is exactly what the I/O thread does.

Regards,

Anthony Liguori