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?
Date: Mon, 10 Sep 2012 19:08:30 +0200 [thread overview]
Message-ID: <20120910170830.GB24448@linux-mips.org> (raw)
In-Reply-To: <20120909193008.GA15157@brightrain.aerifal.cx>
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
next prev parent reply other threads:[~2012-09-10 17:08 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-09 19:30 Is r25 saved across syscalls? Rich Felker
2012-09-10 17:08 ` Ralf Baechle [this message]
2012-09-10 17:22 ` Rich Felker
2012-09-10 18:04 ` David Daney
2012-09-10 18:37 ` Rich Felker
2012-09-11 8:48 ` Ralf Baechle
2012-09-11 13:32 ` Rich Felker
2012-09-11 0:29 ` Maciej W. Rozycki
2012-09-11 1:04 ` Kevin D. Kissell
2012-09-11 1:27 ` David Daney
2012-09-11 2:28 ` Rich Felker
2012-09-11 8:58 ` Ralf Baechle
2012-09-11 8:12 ` Ralf Baechle
2012-09-11 13:44 ` Rich Felker
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