Re: Is r25 saved across syscalls?

[David Daney <ddaney.cavm@gmail.com>]

On 09/10/2012 10:22 AM, Rich Felker wrote:
> On Mon, Sep 10, 2012 at 07:08:30PM +0200, Ralf Baechle wrote:
>> 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.
>
> Is there any documentation of what the kernel does guarantee?

Not really.  The glibc souces can be used as the canonical 
implementation as we cannot break it.  glibc assumes $25 is clobbered.

> All
> existing syscall-making code I've seen depends at least on r4-r7 not
> being clobbered when a signal interrupts a syscall

This is an internal kernel implementation detail. Relying on it in 
userspace is probably not a good idea.

> and sets it up for
> restart (since the arguments still need to be there when it's
> restarted), and seems to also depend on r4-r6 not being clobbered when
> the syscall successfully returns (since they're not listed in the
> clobber list, e.g. in uClibc's inline syscall asm).

Some versions of uClibc's inline syscall asm are buggy.  So they cannot 
be used as an indication of what is supported.

> These are
> requirements beyond the normal function call convention (which does
> not require the callee preserve the values of r4-r7).

I would assume these are clobbered (from glibc sources 
ports/sysdeps/unix/sysv/linux/mips/mips64/n64/sysdep.h):

"$1", "$3", "$10", "$11", "$12", "$13", "$14", "$15", "$24", "$25", 
"hi", "lo"


David Daney