Re: [Qemu-devel] Missing barriers in hw/virtio.c

[Rusty Russell]

On Fri, 26 Aug 2011 15:40:08 +1000, David Gibson <address@hidden> wrote:
> Near the top of hw/virtio.c we have this:
> 
> /* QEMU doesn't strictly need write barriers since everything runs in
>  * lock-step.  We'll leave the calls to wmb() in though to make it
>  obvious for
>  * KVM or if kqemu gets SMP support.
>  * In any case, we must prevent the compiler from reordering the code.
>  * TODO: we likely need some rmb()/mb() as well.
>  */
> 
> #define wmb() __asm__ __volatile__("": : :"memory")
> 
> 
> However, as far as I can tell when using both kvm and io-thread, the
> assertion that barriers aren't necessary just isn't true.  Although it
> probably works on x86 with its strongly ordered model.

That looks very much like it's cribbed from the original lguest code.

Very wrong, please fix!

Thanks,
Rusty.