On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote:
> Seems to give a speedup on my box but I'm less sure about this one. E.g. as
> xchng faster than mfence on all/most intel CPUs? Anyone has an opinion?
Would help if you Cc people who would actually know this :-)
Yes, we've recently established that xchg is indeed faster than mfence
on at least recent machines, see:
lkml.kernel.org/r/ca+55afynbkeuugs9s-q+fly6merba6mjeywwbbe7a5aaqsa...@mail.gmail.com
> +static inline void virtio_store_mb(bool weak_barriers,
> + __virtio16 *p, __virtio16 v)
> +{
> +#ifdef CONFIG_SMP
> + if (weak_barriers)
> + smp_store_mb(*p, v);
> + else
> +#endif
> + {
> + WRITE_ONCE(*p, v);
> + mb();
> + }
> +}
Note that virtio_mb() is weirdly inconsistent with virtio_[rw]mb() in
that they use dma_* ops for weak_barriers, while virtio_mb() uses
smp_mb().
As previously stated, smp_mb() does not cover the same memory domains as
dma_mb() would.
