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

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Date:	Thu, 18 Sep 2014 07:58:17 -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

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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