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

To: Daniel Jacobowitz <dan@debian.org>
Subject: Re: [PATCH, RFC] MIPS: Implement the getcontext API
From: David Daney <ddaney@caviumnetworks.com>
Date: Wed, 04 Mar 2009 08:36:45 -0800
Cc: Brian Foster <brian.foster@innova-card.com>, "Maciej W. Rozycki" <macro@codesourcery.com>, Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, libc-ports@sourceware.org, "Maciej W. Rozycki" <macro@linux-mips.org>
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Daniel Jacobowitz wrote:
On Wed, Mar 04, 2009 at 09:19:28AM +0100, Brian Foster wrote:
 moving the signal trampoline to a vdso (which
 is(? was?) called, maybe misleadingly, ‘vsyscall’,
 on other architectures) is the obvious solution to
 that part of the puzzle.  and yes, it is possible
 to maintain the ABI; the signal trampoline is still
 also put on the stack, and modulo XI, would work if
 used — the trampoline-on-stack is simply not used
 if there is a vdso with the signal trampoline.

That won't quite retain the ABI: you need to make sure everyone
locates it by using the stack pointer instead of the return pc.
Fortunately, GCC uses the return PC only for instruction matching
today.  I have a vague memory it used to use the stack pointer but
this was more reliable.

That is correct. Due to various errata the trampoline cannot always be at a fixed offset to the signal context bits. So we had to use the return PC as you indicate.

David Daney

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