Re: [Qemu-devel] [PATCH] virtio: Make memory barriers be memory barriers

[Paolo Bonzini]

> > No, rmb and wmb need to generate code.
> 
> If they do we'll have to surround each their use with
> ifndef x86 as you suggest later. Which is just messy.

[1 hour later]

I see what you mean now.  You assume there are no accesses to
write-combining memory (of course) or non-temporal load/stores
(because they are accessed only with assembly), so you can
make rmb/wmb no-ops on x86.  I was confused by the kernel
(and liburcu's) choice to use lfence/sfence for rmb/wmb.

Then it's indeed better to move the wmb() defines to
qemu-barrier.h, where they can be made processor-dependent.
S390, it seems, also does not need rmb/wmb.

Paolo