Re: [Qemu-devel] [PATCH] Add a memory barrier to guest memory access functions

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2012-05-21 at 18:58 +1000, Benjamin Herrenschmidt wrote:

> Except that you mostly don't know at that level what you can or cannot
> do, it depends on the caller. We should have the standard accessors do
> it the "safe" way and have performance sensitive stuff do map/unmap, at
> least that's the result of the discussions with Anthony.
> 
> If we can address the virtqueue_map_sg problem, I think we should be
> good, I'll look at it tomorrow. Maybe the right way for now is to remove
> the barrier I added to "map" and only leave the one in _rw

One thing that might alleviate some of your concerns would possibly be
to "remember" in a global (to be replaced with a thread var eventually)
the last transfer direction and use a simple test to chose the barrier,
ie, store + store -> wmb, load + load -> rmb, other -> mb.

But first I'd be curious if some x86 folks could actually measure the
impact of the patch as I proposed it. That would give us an idea of how
bad the performance problem is and how far we need to go to address it.

Cheers,
Ben.