Re: [Qemu-devel] [PATCHv2 3/3] virtio: order index/descriptor reads

[Michael S. Tsirkin]

On Tue, Apr 24, 2012 at 05:40:07PM +0200, Paolo Bonzini wrote:
> >>>> I would be grateful if, instead of fixing the qemu-barrier.h parts of
> >>>> the patches, you picked up the (sole) patch in the atomics branch of
> >>>> git://github.com/bonzini/qemu.git.  The constructs there are more
> >>>> complete than what we have in qemu-barrier.h,
> >>>
> >>> Sorry this is just a bugfix in virtio, don't see a reason to make
> >>> it depend on a wholesale rework of atomics.
> >>
> >> The reason is that your fixes didn't work on PPC, and were suboptimal on
> >> x86
> > 
> > I'll fix PPC but I'll stick to the barriers the way Linux implements
> > them. They pairing rules for these are well documented so we
> > just need to stick to the rules.
> 
> Sure, and smp_rmb() *is* a no-op on Linux:
> 
> #ifdef CONFIG_SMP
> #define smp_mb()        mb()
> #ifdef CONFIG_X86_PPRO_FENCE
> # define smp_rmb()      rmb()        <-- this is an lfence on x86_64
> #else
> # define smp_rmb()      barrier()    <-- this is not
> #endif
> #ifdef CONFIG_X86_OOSTORE
> # define smp_wmb()      wmb()
> #else
> # define smp_wmb()      barrier()
> #endif
> #endif

Hmm, you are right. I'll make it a compiler barrier and add a comment
similar to wmb on x86 explaining that we don't use non-temporals.
Thanks for clarifying this.

-- 
MST