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

[Segher Boessenkool]

On Wed, May 04, 2016 at 02:07:06PM -0500, Segher Boessenkool wrote:
> On Wed, May 04, 2016 at 07:54:03PM +0100, Mark Cave-Ayland wrote:
> > > And the bl clobbers at least r4,r5.
> > 
> > I'm sorry but I still don't understand? __switch_context loads a
> > completely new CPU state from the stack in order to switch context so
> > actually every single register gets clobbered. But we preserve the
> > original link register in a local variable to ensure that we can jump
> > back to where we were upon return.
> 
> If your asm clobbers all regs, the asm needs to say it does.
> 
> So this function clobbers all callee-save registers, but you want it
> to return to the caller anyway?  And that is expected to work?

Mark and I talked on IRC.  It turns out __switch_context does not
switch the context, but also calls the target and switches the context
back.  So there are no such issues; and we don't need to do the call
in asm at all (pure C would work fine), but that is a detail.


Segher