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

[Paolo Bonzini]

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