Re: [PATCH] Don't attach to 'target_changed' observer in regcache

[Mark Kettenis <mark dot kettenis at xs4all dot nl>]

> From: Yao Qi <yao@codesourcery.com>
> Date: Thu, 9 Aug 2012 11:10:16 +0800
> 
> On Wednesday, August 08, 2012 07:53:40 PM Mark Kettenis wrote:
> > > The author (Orjan) tried to support "changing the bank select register
> > > changes the contents (and meaning) for a whole set of other registers."
> > > The requirement is quite specific to Orjan's own port, so the better
> > > solution is to attach a function which invalidates all regcache in
> > > Orjan's backend, instead of doing it in target-independent part.
> > 
> > Banked registers aren't really that exotic.  Especially if you realise
> > that register windows (SPARC, IA-64) are essentially banked registers.
> > 
> 
> Banked register doesn't matter here.  "Changing one special register
> changes the whole set of registers", which is exotic, matters.  Are
> they (banked registers) switched by modifying a certain register?
> Per my few knowledge on SPARC and IA-64, the answer is "No".

Banked registers are by defenition switched by modifying a certain
register I'd say.  And yes, for SPARC by changing %cwp will change the
visible register window and therefore the contents of the %i0-7, %l0-7
and %o0-7 registers.  And changing some bits in the %pstate register
will switch between sets of alternate global registers (%g0-7).  It's
been a while since I last looked at IA-64, but I'm pretty sure it has
some similar mechanisms.  Thinking about this a bit more, PA-RISC is
another architecture that has such a mechanism.  Some of its registers
are shadowed and you can switch between the shadowed and non-shadowed
by modifying a bit in the %psw register.

> The issues of this patch are 1) modifying read-only bits in writable
> register (as Dan pointed out), 2) troubles to Orjan's port.  I can't
> see any other issues of this patch.

And I'm telling you that the "troubles with Orjan's port" are more
general than you think they are.