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

[Benjamin Herrenschmidt]

On Tue, 2012-05-22 at 01:22 +0300, Michael S. Tsirkin wrote:
> > Again, from which originator ? From a given initiator, nothing
> bypasses
> > anything, so the right thing to do here is a full mb(). However, I
> > suspect what you are talking about here is read -responses- not
> > bypassing writes in the direction of the response (ie, the
> "flushing"
> > semantic of reads) which is a different matter.
> 
> No. My spec says:
> A3, A4
> A Posted Request must be able to pass Non-Posted Requests to avoid
> deadlocks. 

An additional note about that one: It only applies obviously if the
initiator doesn't wait for the read response before shooting the write.

Most devices actually do wait. Here too, we would have to explicitly
make sure if what is the right semantic on an individual device basis.

Ben.