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Date:   Mon, 10 Sep 2012 19:08:30 +0200
From:   Ralf Baechle <ralf@linux-mips.org>
To:     Rich Felker <dalias@aerifal.cx>
Cc:     linux-mips@linux-mips.org
Subject: Re: Is r25 saved across syscalls?
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References: <20120909193008.GA15157@brightrain.aerifal.cx>
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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