Re: new barrier type for paravirt (was Re: [PATCH] virtio_ring: use smp_store_mb)

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Dec 20, 2015 at 08:59:44PM +0100, Peter Zijlstra wrote:
> On Sun, Dec 20, 2015 at 05:07:19PM +0000, Andrew Cooper wrote:
> > 
> > Very much +1 for fixing this.
> > 
> > Those names would be fine, but they do add yet another set of options in
> > an already-complicated area.
> > 
> > An alternative might be to have the regular smp_{w,r,}mb() not revert
> > back to nops if CONFIG_PARAVIRT, or perhaps if pvops have detected a
> > non-native environment.  (I don't know how feasible this suggestion is,
> > however.)
> 
> So a regular SMP kernel emits the LOCK prefix and will patch it out with
> a DS prefix (iirc) when it finds but a single CPU. So for those you
> could easily do this.
> 
> However an UP kernel will not emit the LOCK and do no patching.
> 
> So if you're willing to make CONFIG_PARAVIRT depend on CONFIG_SMP or
> similar, this is doable.

One of the uses for virtio is to allow testing an existing kernel on
kvm just by loading a module, and this will break this usecase.

> I don't see people going to allow emitting the LOCK prefix (and growing
> the kernel text size) for UP kernels.

Thinking about this more, maybe __smp_*mb is a better set of names.

The nice thing about it is that we can then have generic code
that does basically

#ifdef CONFIG_SMP
#define smp_mb() __smp_mb()
#else
#define smp_mb() barrier()
#endif 

and reuse this on all architectures.

So instead of a maintainance burden, we are actually
removing code duplication.

-- 
MST