linux-kernel - Re: [PATCH] mips: Save all registers when saving the frame

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Message-ID: <54219BEE.7090309@mvista.com>
Date:	Tue, 23 Sep 2014 11:12:30 -0500
From:	Corey Minyard <cminyard@...sta.com>
To:	Ralf Baechle <ralf@...ux-mips.org>, minyard@....org
CC:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mips: Save all registers when saving the frame

Ping?  I haven't heard anything on this.

Thanks,

-corey

On 09/18/2014 07:58 AM, Corey Minyard wrote:
> On 09/18/2014 04:58 AM, Ralf Baechle wrote:
>> On Tue, Sep 16, 2014 at 04:45:25PM -0500, minyard@....org wrote:
>>
>>> From: Corey Minyard <cminyard@...sta.com>
>>>
>>> The MIPS frame save code was just saving a few registers, enough to
>>> do a backtrace if every function set up a frame.  However, this is
>>> not working if you are using DWARF unwinding, because most of the
>>> registers are wrong.  This was causing kdump backtraces to be short
>>> or bogus.
>>>
>>> So save all the registers.
>> The stratey of partial and full stack frames was developed in '97 to bring
>> down the syscall overhead.  It certaily was very effective - it brought
>> down the syscall latency to the level of Alphas running at much higher
>> clock.
>>
>> That certainly worked well back then for kernel 2.0 / 2.2.  But the syscall
>> code has become much more complex.  Since then support for 64 bit kernels,
>> two 32 bit ABIs running on a 64 bit kernels and numerous features that
>> changed the once simple syscall path have been implemented.  My gut feeling
>> is it might be worth to yank out the whole optimization to see how much
>> code complexity we get rid of in exchange for how much extra syscall
>> latency.
> I"m not sure I understand.  From what I can tell, this code is only
> called by
> things that print stack traces, kdb, and kexec/kdump.  So it shouldn't be in
> any normal syscall path.
>
> This patch will currently only help kdump, but it will be necessary if
> anyone
> adds MIPS support for DWARF unwinding for stack traces.  And you'd have
> to fix some things in context switching, too, I think.
>
> From what I can tell the partial save for syscalls is a good idea.  You
> don't have
> to save half the registers and it doesn't affect tracebacks, kdump, or
> anything else
> like that.
>
> -corey

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