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From: Juli Mallett <jmallett@FreeBSD.org>
Date: Thu, 28 Jan 2010 22:30:42 -0800
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To: Randall Stewart <rrs@lakerest.net>
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Cc: freebsd-mips@freebsd.org, Neel Natu <neelnatu@gmail.com>
Subject: Re: Code review: groundwork for SMP
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On Thu, Jan 28, 2010 at 21:47, Randall Stewart <rrs@lakerest.net> wrote:
> Do all of the mips processors have this scratch registers (I guess
> I should scope that.. do the ones we care about have a scratch
> register).. or is this an optional feature.

Not anything like all or even all interesting ones.  There are some
registers you could try abusing that are widely-available but mostly
unused, but most of them would require such a degree of discipline
that they're not worth it.  (Hell, you could use badvaddr as long as
you refreshed it after every TLB miss.)

> But I wonder how it compares to a indexed access that doing a
>
> pcpup = &pcpu[getcpuid()];

The problem is get "getcpuid" is very slow on some systems.  If you
have a really quick way of getting it, this isn't too bad, but my
understanding is that that's one of the key reasons to avoid the array
approach.

Juli.