On Thu, 2012-05-17 at 10:07 +1000, Benjamin Herrenschmidt wrote: .../...
> Also, should I make the barrier conditional to kvm_enabled() ? IE. It's > pointless in full emulation and might actually be a performance hit on > something already quite slow... Finally ... something like smp_mb() in qemu will turn into a lock op or an mfence on x86, ie not a nop. That means overhead from today's implementation, which leads to the question ... is today implementation correct ? IE. Is a barrier needed on x86 as well or not ? If not (I'm trying to figure out why exactly does x86 have a barrier in the first place and when it's in order), then I might add a new barrier type in qemu-barriers.h, something like dma_mb(), and define it as a nop on x86, a lwsync or sync (still thinking about it) on ppc, and __sync_synchronize() on unknown archs. Any x86 guru around cares to explain me what exactly is the x86 memory model and when does it need barriers ? Cheers, Ben.
