[parisc-linux] ELF ABI Documents

[parisc-linux] ELF ABI Documents

[Jeff Bailey]

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I'm on a bit of a quest for accurate documentation.

This started when I was starting to hack klibc and had to go digging for
a bunch of information in a pile of different documents.  What I'd like
to do is write the LSB document for parisc so that there's a single
accurate place to refer to for calling conventions / syscall
information, etc.

It's most common in the LSB specs to refer to the ELF psabi document
where possible.  http://refspecs.freestandards.org/elf/elf-pa.pdf seems
to be a copy of the psabi document, but when checking with some folks,
it looks like the document isn't a reflection on reality for
parisc-linux.  The question is, should the document be updated to
reflect reality, should we slowly change to match the document, or
should I just not refer to the psabi document much when writing the LSB
document?

Tks,
Jeff Bailey

-- 
Although when you're in the situation that RMS is telling you that
you're being too ideological about freedom, maybe, just maybe, it's
true.
- Matthew Wilcox

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Re: [parisc-linux] ELF ABI Documents

[Randolph Chung]

> It's most common in the LSB specs to refer to the ELF psabi document
> where possible.  http://refspecs.freestandards.org/elf/elf-pa.pdf seems
> to be a copy of the psabi document, but when checking with some folks,
> it looks like the document isn't a reflection on reality for
> parisc-linux.  The question is, should the document be updated to
> reflect reality, should we slowly change to match the document, or
> should I just not refer to the psabi document much when writing the LSB
> document?

First of all, I want to say that such a document would be wonderful to have!

I think the ELF-PA document corresponds mostly with what we do on
parisc-linux. There are some things we don't implement (e.g. I don't
think we have PCREL14WR/PCREL14DR), and we have made some enhancements
in other areas (e.g. TLS relocations). The only thing that I know of
where we don't follow the spec is how we handle SEGREL32 - we don't use
a SEGBASE relocation to set the segment base, but rely on other
information from the ELF headers.

I don't think the ELF spec talks about calling conventions and syscall
conventions though. For that, parisc-linux mostly follows the SOM spec
for calling conventions
(http://ftp.parisc-linux.org/docs/arch/pa-runtime-32-SOM.pdf)

A document about hppa ABI will not be complete without some discussion
about the hppa unwinding mechanism too.

my 2 cents :)
randolph
-- 
Randolph Chung
Debian GNU/Linux Developer, hppa/ia64 ports
http://www.tausq.org/
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Re: [parisc-linux] ELF ABI Documents

[Jeff Bailey]

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Le mardi 18 avril 2006 à 11:00 +0800, Randolph Chung a écrit :

> I don't think the ELF spec talks about calling conventions and syscall
> conventions though. For that, parisc-linux mostly follows the SOM spec
> for calling conventions
> (http://ftp.parisc-linux.org/docs/arch/pa-runtime-32-SOM.pdf)

Right.  This is where the LSB document comes it.  It does wind up
defining those for Linux at least.  The "Mostly follows SOM" is largely
what's inspired me on this quest.  I'd love to see a definitive document
that defines this and am willing to go through the effort to do it.

Tks,
Jeff Bailey

-- 
Although when you're in the situation that RMS is telling you that
you're being too ideological about freedom, maybe, just maybe, it's
true.
- Matthew Wilcox

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Re: [parisc-linux] ELF ABI Documents

[John David Anglin]

> > I don't think the ELF spec talks about calling conventions and syscall
> > conventions though. For that, parisc-linux mostly follows the SOM spec
> > for calling conventions
> > (http://ftp.parisc-linux.org/docs/arch/pa-runtime-32-SOM.pdf)
> 
> Right.  This is where the LSB document comes it.  It does wind up
> defining those for Linux at least.  The "Mostly follows SOM" is largely
> what's inspired me on this quest.  I'd love to see a definitive document
> that defines this and am willing to go through the effort to do it.

I think what is needed is a SYSV PS ABI for PA-RISC.  It probably shouldn't
be an LSB document.  Typically, the LSB only adopts a subset of any
particular specification, and in the case of the PS ABIs, references
the documents for the various processors currently specified by the
LSB.  The IA64 document is probably the closest to what's needed for
PA-RISC although it can be simplified.  It was written jointly by HP
and Intel.

It probably would be best to obtain the support of HP in doing this.
One of the criticisms of the LSB is that it simply documents what is
and in large measure it just refers to other specifications.
Whereas, specifications such as POSIX have been written by people
aware of the subtle tradeoffs between different implementations
and they try to actually specify an implementation.  If it's
possible to obtain full community support, it's more likely that the
document would be useful and a guide to implementors.  However,
just documenting what is would be useful.

The PA-RISC ABI isn't completely static.  For example, I changed the
passing of complex values in GCC 4.1 so that they are now treated in
the same manner as aggregates.  I think it's also fair to say that
the TLS implementation is far from finalized.

As noted, the HP ELF documents aren't completely up to date and
they don't fully match the current headers provided with HP-UX.
For example, there are now note sections for PA-RISC.  It would be
useful if these documents could be updated as well but this would
require permission from HP because of the copyright issues.

The HP ELF implementation differs in some significant ways from
that in linux.  Areas that come immediately to mind are weak symbol
support, program startup, TLS implementation, memory model, debug
support, etc.

>>From my perspective, a well written PS ABI for PA-RISC would certainly
be useful.  I suspect HP dropped work in this area because of its
decision to phase out PA-RISC.  This is certainly a challenging
project but maybe once the scope is defined the rest will follow.
If you work fast, you might beat implementation of a 64-bit PA-RISC
runtime for linux!

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)
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Re: [parisc-linux] ELF ABI Documents

[Carlos O'Donell]

On 4/18/06, John David Anglin <dave@hiauly1.hia.nrc.ca> wrote:
> I think what is needed is a SYSV PS ABI for PA-RISC.  It probably shouldn=
't
> be an LSB document.  Typically, the LSB only adopts a subset of any
> particular specification, and in the case of the PS ABIs, references
> the documents for the various processors currently specified by the
> LSB.  The IA64 document is probably the closest to what's needed for
> PA-RISC although it can be simplified.  It was written jointly by HP
> and Intel.

The SYSV psABI for HP PA-RISC is going to be a very messy thing to start wi=
th.
I whole heartedly recommend that Jeff begin work by writing the ELF
psABI for hppa and perhaps after that moving towards the SYSV psABI.
The LSB documentation will then be a reference to the SYSV psABI and
the ELF psABI documents.

So Jeff, care to whip up a document that is a copy of the elf-pa.pdf and
we can start commenting on what's wrong with the document?

What document format are you going to use? I recommend openoffice or tex.

List of the relocs we handle:
binutils/include/elf/hppa.h
  This file lists our relocs and their values. With comments about
their processing.
binutils/bfd/elf-hppa.h
  This file contains the relocs howto showing if some are unimplemented.
  (elf_hppa_final_write_processing): Contains flag settings for the elf hea=
der.

Cheers,
Carlos.
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Re: [parisc-linux] ELF ABI Documents

[John David Anglin]

> It's most common in the LSB specs to refer to the ELF psabi document
> where possible.  http://refspecs.freestandards.org/elf/elf-pa.pdf seems

The latest version is here:
http://ftp.parisc-linux.org/docs/arch/elf-pa-hp.pdf

HP and Intel also developed a generic specification for the ELF-64 object
file format, and a version containing HP and HP-UX extensions.

> to be a copy of the psabi document, but when checking with some folks,
> it looks like the document isn't a reflection on reality for
> parisc-linux.  The question is, should the document be updated to
> reflect reality, should we slowly change to match the document, or
> should I just not refer to the psabi document much when writing the LSB
> document?

None of the above mentioned documents appear to contain a copyright
notice.  However, this may no longer be an absolute requirement under
US copyright law.  In as much as a linux document is likely going to
have to utilize material from the above, it probably would be wise to
get the written approval from HP before proceeding.  It might also be
necessary to get Intel's approval if the document also attempts to
specify ELF for 64-bit linux.  Of course if you just reference these
specifications and point out differences, this problem can be avoided.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)
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Re: [parisc-linux] ELF ABI Documents

[Matt Taggart]

"John David Anglin" writes...

> One of the criticisms of the LSB is that it simply documents what is
> and in large measure it just refers to other specifications.

1) The LSB is a trailing-edge standard, they don't "invent" standards, they 
just document the standards that are accepted and in use. The work coming up 
with new standards is being done elsewhere. (X.org and freedesktop.org for 
example)
2) Referring to other specs is a good thing, why reinvent the wheel?

> Whereas, specifications such as POSIX have been written by people
> aware of the subtle tradeoffs between different implementations
> and they try to actually specify an implementation.

To be fair, the LSB is in a hard position. On one hand you have application 
developers demanding an implementation "We want a solid predictable 
implementation that we can test on" and on the other you have the Linux 
distros screaming "Don't take away our right to innovate! Just stick to 
documenting interfaces and we'll make sure our implementations are compliant 
with them". What the LSB has done to try and balance in the middle is define a 
"sample implementation" rather than a "reference implementation". The LSB gets 
asked once a month "can't you just get all the Linux distros to agree on an 
implementation, or declare one of them the winner that everyone should use?" 
IMO the UnitedLinux, DCC, and UserLinux consortiums were all created with that 
as a goal.

There is also some communication between the LSB workgroup and the Austin 
Group and they are both benefitting from the other's work.

> If it's
> possible to obtain full community support, it's more likely that the
> document would be useful and a guide to implementors.  However,
> just documenting what is would be useful.

Here, here! And don't worry about doing the work via the LSB, just produce the 
standard and the LSB will adopt it when it makes sense.

-- 
Matt Taggart
matt@lackof.org


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Re: [parisc-linux] ELF ABI Documents

[John David Anglin]

> There is also some communication between the LSB workgroup and the Austin 
> Group and they are both benefitting from the other's work.

I'm aware of this and the efforts to align POSIX and the LSB where
differences exist.  The majority of areas where differences exist
between POSIX and linux are resolved in current implementations.
Thus, it appears that the standards can be aligned in future updates
without too much difficulty.

FWIW, I personally believe that the development of the LSB was a
useful effort.  However, as you know, one of Austin Group participants
believes the LSB to be useless...   Time will tell.

> > If it's
> > possible to obtain full community support, it's more likely that the
> > document would be useful and a guide to implementors.  However,
> > just documenting what is would be useful.
> 
> Here, here! And don't worry about doing the work via the LSB, just produce the 
> standard and the LSB will adopt it when it makes sense.

It would be a significant achievement if production of these documents
allowed PA-RISC to become part of the LSB standard!  I think it fair to
say that almost all (all?) of the processor specific documents referenced
by the LSB were developed by commercial entities rather than the open source
community at large.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)
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