[parisc-linux] xchg implementation

[Matthew Wilcox]

On Mon, Nov 01, 1999 at 02:03:05PM -0700, LaMont Jones wrote:
> > This is a uniprocessor optimisation of the xchg() case where it's _not_
> > exchanging with zero.  What I need is a spare CR register.  As far as I
> > can see from the comments, CR28-30 are currently reserved; leaving CRs
> > 24, 25 and 31 available.  Unless someone's using one of them without
> > documenting it somewhere...?  That really does need to be written down
> > somewhere.
> 
> IIRC, CR24 and 25 are readable from user space (not writable), and can be
> quite useful for thread specific data, thread id, etc...  It might do to
> leave them available for such a use.

You're thinking of CR26 and 27 (ref: Kane's PA-RISC 2.0 Architecture,
page 2-17).  CR31 _is_ used, I didn't read carefully enough:

$ find arch/parisc/ -name \*.[chS] |xargs grep ctl |grep 31
arch/parisc/kernel/interruption.S:      mtctl           29,31
arch/parisc/kernel/interruption.S:      mtctl           29,31
arch/parisc/kernel/interruption.S:      mtctl           29,31
arch/parisc/kernel/syscall.S:   mfctl   31, %r27                        ; get the kernel dp

cr26,27 are used, but only in a debug macro in head.S.

-- 
Matthew Wilcox <willy@bofh.ai>
"Windows and MacOS are products, contrived by engineers in the service of
specific companies. Unix, by contrast, is not so much a product as it is a
painstakingly compiled oral history of the hacker subculture." - N Stephenson