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Date: Thu, 03 Nov 2011 12:50:14 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Multiple instances of Qemu on Windows multicore
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To: Avi Kivity <avi@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org, Fabien Chouteau <chouteau@adacore.com>

On 11/03/2011 11:29 AM, Avi Kivity wrote:
> >  It would ensure that two mutators wouldn't run concurrently.  In some
> >  sense, signal-safe code could then be considered thread-safe too.
>
> How so?  The scheduler can switch between the two threads on every
> instruction.

In general signal-safe is more stringent than thread-safe, but with two 
exceptions: memory barriers and locked memory access.  On x86 (implied 
by Windows...) you might also assume that the compiler will generate 
arithmetic operations with a memory destination, which makes code like

     void cpu_interrupt(CPUState *env, int mask)
     {
         env->interrupt_request |= mask;          /* <--- this */
         cpu_unlink_tb(env);
     }

signal-safe in practice---and even "thread-safe" on non-SMP systems. 
It's a huge assumption though, and I don't think it should be assumed 
anymore.  With iothread the architecture of the QEMU main loop is anyway 
completely different.

Paolo