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

[Michael S. Tsirkin]

On Mon, May 21, 2012 at 05:31:06PM -0500, Anthony Liguori wrote:
> On 05/21/2012 05:26 PM, Benjamin Herrenschmidt wrote:
> >On Mon, 2012-05-21 at 17:18 -0500, Anthony Liguori wrote:
> >>But this isn't what this series is about.
> >>
> >>This series is only attempting to make sure that writes are ordered
> >>with respect
> >>to other writes in main memory.
> >
> >Actually, it applies to both reads and writes. They can't pass each
> >other either and that can be fairly important.
> 
> That's fine but that's a detail of the bus.
> 
> >It's in fact the main contention point because if it was only writes we
> >could just use wmb and be done with it (that's a nop on x86).
> >
> >Because we are trying to order everything (and specifically store
> >followed by a load), we need a full barrier which is more expensive on
> >x86.
> 
> I think the thing to do is make the barrier implemented in the dma
> API and allow it to be overridden by the bus.  The default
> implementation should be a full barrier.

I think what's called for is what Ben proposed: track
last transaction and use the appropriate barrier.

> If we can establish that the bus guarantees a weaker ordering
> guarantee, a bus could override the default implementation and do
> something weaker.
> 
> Regards,
> 
> Anthony Liguori

OK. Just not another level of indirect function callbacks please.  Make
it a library so each bus can do the right thing.  There are not so many
buses.

-- 
MST