[OpenBIOS] [PATCH 2/4] ppc: add new context handler

Segher Boessenkool segher at kernel.crashing.org
Wed May 4 17:31:17 CEST 2016

On Tue, May 03, 2016 at 10:12:09PM +0100, Mark Cave-Ayland wrote:
> >>> Oh, yeah, that's another problem: a call can clobber all volatile
> >>> (caller-save) registers.  The best solution is to write this whole
> >>> thing in actual assembler, instead of fighting the compiler to get
> >>> just the assembler you want ;-)
> >>
> >> Heh actually the bug was a missing volatile qualifier from the __context
> >> pointer which is interesting since this code is used almost boilerplate
> >> for other architectures - I'll spin up a patch to update the other archs
> >> once this has gone in. The v2 version of the patchset contains this
> >> correction though.
> > 
> > That is just a workaround.  It might well work, and it can also break
> > at any time.
> Can you explain further? The v2 patch uses a single register marked as
> clobber for mflr/mtlr and the volatile modifier for __context ensures
> that the old value being written to the stack doesn't get optimised away
> by the compiler. What else am I missing?

A normal (ABI-following) function can clobber all of r9..r12, so that
"bl" instruction can, too.  If the function it calls is special (so at
least written as assembler), things are different of course.


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