Re: [Xen-devel] [PATCH v2 20/32] metag: define __smp_xxx

[James Hogan <james.hogan@xxxxxxxxxx>]

On Mon, Jan 04, 2016 at 04:30:36PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 04, 2016 at 03:25:58PM +0000, James Hogan wrote:
> > It is used along with the metag specific __global_lock1() (global
> > voluntary lock between hw threads) whenever a write is performed, and by
> > smp_mb/smp_rmb to try to catch other cases, but I've never been
> > confident this fixes every single corner case, since there could be
> > other places where multiple CPUs perform unsynchronised writes to the
> > same memory location, and expect cache not to become incoherent at that
> > location.
> 
> Ah, yuck, I thought blackfin was the only one attempting !coherent SMP.
> And yes, this is bound to break in lots of places in subtle ways. We
> very much assume cache coherency for SMP in generic code.

Well, its usually completely coherent, its just a bit dodgy in a
particular hardware corner case, which was pretty hard to hit, even
without these workarounds.

> 
> > It seemed to be sufficient to achieve stability however, and SMP on Meta
> > Linux never made it into a product anyway, since the other hw thread
> > tended to be used for RTOS stuff, so it didn't seem worth extending the
> > generic barrier API for it.
> 
> *phew*, should we take it out then, just to be sure nobody accidentally
> tries to use it then?

SMP support on this SoC you mean? I doubt it'll be a problem tbh, and
it'd work fine in QEMU when emulating this SoC, so I'd prefer to keep it
in.

Cheers
James

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