Re: [PATCH, RFC] MIPS: Implement the getcontext API

[David Daney <ddaney@caviumnetworks.com>]

Adding Richard S. as he may be interested...

Maciej W. Rozycki wrote:
> On Tue, 3 Mar 2009, David Daney wrote:
> 
>> Note the libgcc currently makes the assumption that the layout of the stack
>> for signal handlers is fixed.  The DWARF2 unwinder needs this information to
>> be able to unwind through signal frames (see gcc/config/mips/linux-unwind.h),
>> so it is already a de facto part of the ABI.
> 
>  I do hope it was agreed upon at some point.

As with many things, there was no formal agreement.

> I certainly cannot recall a 
> discussion at the linux-mips list, but I did not always follow it closely 
> enough either, so I may have missed the discussion.

http://www.linux-mips.org/cgi-bin/mesg.cgi?a=linux-mips&i=473957B6.3030202%40avtrex.com
http://www.linux-mips.org/cgi-bin/mesg.cgi?a=linux-mips&i=4739CCD6.2080306%40avtrex.com

> The interface is 
> meant to be internal to Linux, so the usual rule of volatility apply.  The 
> structure is not defined in a header even.
> 

Certainly it started out that way, but if the kernel doesn't supply 
DWARF2 unwind tables for its signal trampolines (which it currently does 
not), then I think using the structures is the only way for user-space 
applications to unwind through signal trampolines.

I was pointing this out not as any type of objection to your plan, but 
as further support for formalizing the interfaces.

>>>  Furthermore I am requesting that the kernel recognises the special meaning
>>> of the value of one stored in the slot designated for the $zero register and
>>> never places such a value itself there.
>> Seems reasonable to me as currently a zero is unconditionally stored there.
> 
>  It is, but is should be architected, not assumed.  Also contexts built 
> with the *context() functions are meant to be usable by them only -- 
> software will still be able to assume the value in the slot when 
> constructed by the kernel.
> 

Agreed.

Thanks for working on this,
David Daney