Re: [PATCH v4 2/2] arm64: enable context tracking

[Will Deacon <will.deacon@arm.com>]

On Fri, May 23, 2014 at 04:55:44PM +0100, Mark Rutland wrote:
> On Fri, May 23, 2014 at 03:51:07PM +0100, Catalin Marinas wrote:
> > On Fri, May 23, 2014 at 01:11:38AM +0100, Kevin Hilman wrote:
> > I haven't checked all the code paths but at least for pushing onto the
> > stack we must keep it 16-bytes aligned (architecture requirement).
> 
> Sure -- if modifying the stack we need to push/pop pairs of registers to
> keep it aligned. It might be better to use xzr as the dummy value in
> that case to make it clear that the value doesn't really matter.
> 
> That said, ct_user_enter is only called in kernel_exit before we restore
> the values off the stack, and the only register I can spot that we need
> to preserve is x0 for the syscall return value. I can't see x1 or x2
> being used any more specially than the rest of the remaining registers.
> Am I missing something, or would it be sufficient to do the following?
> 
> push	x0, xzr
> bl	context_tacking_user_enter
> pop	x0, xzr

... and if that works, then why are we using the stack instead of a
callee-saved register?

Will