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Date: Fri, 18 Jan 2019 17:12:43 -0600
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH 2/4] powerpc/64s: Add slb_full_bitmap rather than
 hard-coding U32_MAX
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On Fri, Jan 18, 2019 at 11:28:24PM +1100, Michael Ellerman wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> 
> > On Thu, Jan 17, 2019 at 11:13:26PM +1100, Michael Ellerman wrote:
> >> The recent rewrite of the SLB code into C included the assumption that
> >> all CPUs we run on have at least 32 SLB entries. This is currently
> >> true but a bit fragile as the SLB size is actually defined by the
> >> device tree and so could theoretically change at any time.
> >
> > It also is guaranteed by the architecture, since at least 2.02, FWIW.
> 
> True. Actually 2.00 says at least 32.
> 
> Unfortunately we don't live in a world where "the architecture
> guarantees it" has any bearing on reality :)

It's a pretty strong hint.  I don't remember any hardware where it is not
true, either.  (That might be selective memory ;-) )

> But given it *should* always be at least 32 maybe I should optimise for
> that case. We could use a static key to skip the U32_MAX comparison and
> go down the else path.

Ah that sounds like a good idea :-)


Segher