[Qemu-devel] Re: [PATCH 2/7] Enable I/O thread and VNC threads by default

[Anthony Liguori]

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