[RFC] virtio: use mandatory barriers for remote processor vdevs

[Rusty Russell]

On Thu, 1 Dec 2011 10:12:37 +0200, "Michael S. Tsirkin" <mst at redhat.com> wrote:
> On Thu, Dec 01, 2011 at 12:58:59PM +1030, Rusty Russell wrote:
> > On Thu, 1 Dec 2011 01:13:07 +0200, "Michael S. Tsirkin" <mst at redhat.com> wrote:
> > > For x86, stores into memory are ordered. So I think that yes, smp_XXX
> > > can be selected at compile time.
> > > 
> > > So let's forget the virtio strangeness for a minute,
> > 
> > Hmm, we got away with light barriers because we knew we were not
> > *really* talking to a device.  But now with virtio-mmio, turns out we
> > are :)
> 
> You think virtio-mmio this issue too?  It's reported on remoteproc...

I think any non-virtual, non-PCI device has to worry about it.  Perhaps
all virtio-mmio are virtual (at this point).

I'm tempted to say we want permission from the device to do relaxed
barriers (so I don't have to worry about it!)

> > I'm really tempted to revert d57ed95 for 3.2, and we can revisit this
> > optimization later if it proves worthwhile.
> 
> Generally it does seem the best we can do for 3.2.
> 
> Given it's rc3, I'd be a bit wary of introducing regressions - I'll try
> to find some real setups (as in - not my laptop) to run some benchmarks
> on, to verify there's no major problem.
> I hope I can report on this in about a week from now - want to hold onto this meanwhile?

Yep, no huge hurry.  Thanks!

Cheers,
Rusty.