Re: [Qemu-devel] Multiple instances of Qemu on Windows multicore

[Fabien Chouteau]

On 03/11/2011 12:50, Paolo Bonzini wrote:
> 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. 

What can we do to improve that?

>
> With iothread the architecture of the QEMU main loop is anyway completely 
> different.
> 

Are you saying that things are better or worst with iothread?

-- 
Fabien Chouteau