Re: Is r25 saved across syscalls?

[Ralf Baechle <ralf@linux-mips.org>]

On Sun, Sep 09, 2012 at 03:30:08PM -0400, Rich Felker wrote:

> The kernel syscall entry/exit code seems to always save and restore
> r25. Is this stable/documented behavior I can rely on? If there's a
> reason it _needs_ to be preserved, knowing that would help convince me
> it's safe to assume it will always be done. The intended usage is to
> be able to make syscalls (where the syscall # is not a constant that
> could be loaded with lwi) without a stack frame, as in "move $2,$25 ;
> syscall".

The basic design idea is that syscalls use a calling convention similar
to subroutine calls.  $25 is $t9, so a temp register which is callee saved.

So if the kernel is saving $t9 and you've been relying on that, consider
yourself lucky - there's not guarantee for that.

  Ralf