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Date: Mon, 22 Aug 2011 09:09:33 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] main: force enabling of I/O thread
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Anthony Liguori <aliguori@us.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Blue Swirl <blauwirbel@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Aurelien Jarno <aurelien@aurel32.net>

On 08/22/2011 08:50 AM, Peter Maydell wrote:
> On 22 August 2011 14:24, Anthony Liguori<aliguori@us.ibm.com>  wrote:
>> Enabling the I/O thread by default seems like an important part of declaring
>> 1.0.  Besides allowing true SMP support with KVM, the I/O thread means that the
>> TCG VCPU doesn't have to multiplex itself with the I/O dispatch routines which
>> currently requires a (racey) signal based alarm system.
>
> Even with iothread it's still signal based (and still racy) -- the only way
> to get a thread currently executing TCG code to stop doing so is to send it
> a signal.

What I meant is that we use SIGALRM to effectively do preemptive 
multiple tasking between the VCPU thread and the I/O thread.

We still need to use signals with the I/O thread because we run the two 
in lock step.  The race I was referring to with SIGALRM has to do with a 
guest disabling timer interrupts and any other event source.  It'll 
cause a live lock in TCG.

The only way to fix this is by moving to the I/O thread or setting SIGIO 
on every fd.  Since every fd doesn't support SIGIO, I/O thread is really 
the only correct solution.

Regards,

Anthony Liguori

>
> -- PMM
>