RFC: Adding arch-specific user ABI documentation in linux-man

RFC: Adding arch-specific user ABI documentation in linux-man

[Dave Martin]

Hi all,

I considering trying to plug some gaps in the arch-specific ABI
documentation in the linux man-pages, specifically for arm64 (and
possibly arm, where compat means we have some overlap).

For arm64, there are now significant new extensions (Pointer
authentication, SVE, MTE etc.)  Currently there is some user-facing
documentation mixed in with the kernel-facing documentation in the
kernel tree, but this situation isn't ideal.

Do you have an opinion on where in the man-pages documentation should be
added, and how to structure it?


Affected areas include:

 * exec interface
 * aux vector, hwcaps
 * arch-specific signals
 * signal frame
 * mmap/mprotect extensions
 * prctl calls
 * ptrace quirks and extensions
 * coredump contents


Not everything has an obvious home in an existing page, and adding
specifics for every architecture could make some existing manpages very
unwieldy.

I think for some arch features, we really need some "overview" pages
too: just documenting the low-level details is of limited value
without some guide as to how to use them together.


Does the following sketch look reasonable?

 * man7/arm64.7: new page: overview of arm64-specific ABI extensions

 * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
   overview of arm64 SVE ABI

 * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
   arm64 ptrace extensions

 * man2/mmap.2: extend with arm64-specific flags (only two flags, so we
   add them to the existing man page rather than creating a new one).

etc.


Ideally, I'd like to adopt a pattern that other arches can follow.

Thoughts?

Cheers
---Dave

AW: Adding arch-specific user ABI documentation in linux-man

[Walter Harms]

Hi Dave,
you are pointing to an (IMHO) interesting question.
How to document different CPUs ?
Given that an operating system should hide the different CPU's using
CPU specific features should be used sparsely at best.

the easy part are adds-on like flags for prctrl etc. simply add it to the page.

Other things should go to a cpu specific pages (can of worms). The problem will
be to keep that small but informative. I have no idea about the level of detail
(and i have worked with a range of CPUs) that could be interesting for a programmer.
An of cause every other CPU now needs also a page.

jm2c
 wh
________________________________________
Von: linux-man-owner@vger.kernel.org <linux-man-owner@vger.kernel.org> im Auftrag von Dave Martin <Dave.Martin@arm.com>
Gesendet: Montag, 4. Mai 2020 17:32:35
An: Michael Kerrisk
Cc: Catalin Marinas; Will Deacon; Vincenzo Frascino; linux-man@vger.kernel.org; linux-arch@vger.kernel.org; linux-arm-kernel@lists.infradead.org
Betreff: RFC: Adding arch-specific user ABI documentation in linux-man

Hi all,

I considering trying to plug some gaps in the arch-specific ABI
documentation in the linux man-pages, specifically for arm64 (and
possibly arm, where compat means we have some overlap).

For arm64, there are now significant new extensions (Pointer
authentication, SVE, MTE etc.)  Currently there is some user-facing
documentation mixed in with the kernel-facing documentation in the
kernel tree, but this situation isn't ideal.

Do you have an opinion on where in the man-pages documentation should be
added, and how to structure it?


Affected areas include:

 * exec interface
 * aux vector, hwcaps
 * arch-specific signals
 * signal frame
 * mmap/mprotect extensions
 * prctl calls
 * ptrace quirks and extensions
 * coredump contents


Not everything has an obvious home in an existing page, and adding
specifics for every architecture could make some existing manpages very
unwieldy.

I think for some arch features, we really need some "overview" pages
too: just documenting the low-level details is of limited value
without some guide as to how to use them together.


Does the following sketch look reasonable?

 * man7/arm64.7: new page: overview of arm64-specific ABI extensions

 * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
   overview of arm64 SVE ABI

 * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
   arm64 ptrace extensions

 * man2/mmap.2: extend with arm64-specific flags (only two flags, so we
   add them to the existing man page rather than creating a new one).

etc.


Ideally, I'd like to adopt a pattern that other arches can follow.

Thoughts?

Cheers
---Dave

Re: Adding arch-specific user ABI documentation in linux-man

[Dave Martin]

On Tue, May 05, 2020 at 07:45:32AM +0000, Walter Harms wrote:
> Hi Dave,
> you are pointing to an (IMHO) interesting question.
> How to document different CPUs ?
> Given that an operating system should hide the different CPU's using
> CPU specific features should be used sparsely at best.

Agreed!  But there are real situations where cpu specifics can't be
avoided, and having documentation will help people to use those features
correctly.

> the easy part are adds-on like flags for prctrl etc. simply add it to the page.

For prctl, that makes sense (it's a jumble of arch specifics already).

But would it be considered a problem if the ptrace page, say, becomes
80% arch-specific stuff?  That page is cumbersome enough already...

> Other things should go to a cpu specific pages (can of worms). The problem will
> be to keep that small but informative. I have no idea about the level of detail
> (and i have worked with a range of CPUs) that could be interesting for a programmer.
> An of cause every other CPU now needs also a page.

Fair enough.

Cheers
---Dave

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Will Deacon]

Hi Dave,

On Mon, May 04, 2020 at 04:32:35PM +0100, Dave Martin wrote:
> I considering trying to plug some gaps in the arch-specific ABI
> documentation in the linux man-pages, specifically for arm64 (and
> possibly arm, where compat means we have some overlap).
> 
> For arm64, there are now significant new extensions (Pointer
> authentication, SVE, MTE etc.)  Currently there is some user-facing
> documentation mixed in with the kernel-facing documentation in the
> kernel tree, but this situation isn't ideal.
> 
> Do you have an opinion on where in the man-pages documentation should be
> added, and how to structure it?
> 
> 
> Affected areas include:
> 
>  * exec interface
>  * aux vector, hwcaps
>  * arch-specific signals
>  * signal frame
>  * mmap/mprotect extensions
>  * prctl calls
>  * ptrace quirks and extensions
>  * coredump contents
> 
> 
> Not everything has an obvious home in an existing page, and adding
> specifics for every architecture could make some existing manpages very
> unwieldy.
> 
> I think for some arch features, we really need some "overview" pages
> too: just documenting the low-level details is of limited value
> without some guide as to how to use them together.
> 
> 
> Does the following sketch look reasonable?
> 
>  * man7/arm64.7: new page: overview of arm64-specific ABI extensions
> 
>  * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
>    overview of arm64 SVE ABI
> 
>  * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
>    arm64 ptrace extensions

Michael has been nagging me on and off about that for, what, 10 years now?
I would therefore be very much in favour of having our ptrace extensions
documented!

We could even put this stuff under Documentation/arm64/man/ if it's deemed
too CPU-specific for the man-pages project, but my preference would still
be for it to be hosted there alongside all the other man pages.

Will

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Dave Martin]

On Tue, May 05, 2020 at 11:44:55AM +0100, Will Deacon wrote:
> Hi Dave,
> 
> On Mon, May 04, 2020 at 04:32:35PM +0100, Dave Martin wrote:
> > I considering trying to plug some gaps in the arch-specific ABI
> > documentation in the linux man-pages, specifically for arm64 (and
> > possibly arm, where compat means we have some overlap).
> > 
> > For arm64, there are now significant new extensions (Pointer
> > authentication, SVE, MTE etc.)  Currently there is some user-facing
> > documentation mixed in with the kernel-facing documentation in the
> > kernel tree, but this situation isn't ideal.
> > 
> > Do you have an opinion on where in the man-pages documentation should be
> > added, and how to structure it?
> > 
> > 
> > Affected areas include:
> > 
> >  * exec interface
> >  * aux vector, hwcaps
> >  * arch-specific signals
> >  * signal frame
> >  * mmap/mprotect extensions
> >  * prctl calls
> >  * ptrace quirks and extensions
> >  * coredump contents
> > 
> > 
> > Not everything has an obvious home in an existing page, and adding
> > specifics for every architecture could make some existing manpages very
> > unwieldy.
> > 
> > I think for some arch features, we really need some "overview" pages
> > too: just documenting the low-level details is of limited value
> > without some guide as to how to use them together.
> > 
> > 
> > Does the following sketch look reasonable?
> > 
> >  * man7/arm64.7: new page: overview of arm64-specific ABI extensions
> > 
> >  * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
> >    overview of arm64 SVE ABI
> > 
> >  * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
> >    arm64 ptrace extensions
> 
> Michael has been nagging me on and off about that for, what, 10 years now?
> I would therefore be very much in favour of having our ptrace extensions
> documented!
> 
> We could even put this stuff under Documentation/arm64/man/ if it's deemed
> too CPU-specific for the man-pages project, but my preference would still
> be for it to be hosted there alongside all the other man pages.

Heh, perhaps we could build that into the kernel and mount it somewhere.


Seriously though,

I guess I can start off with straightforward small things for which the
documentation has an obvious home (like prctls[*]) and then move on to
the bigger stuff like ptrace.

If people start shouting about a page getting too big or messy I can try
to split it up.

Make sense?

Cheers
---Dave


[*] "straightforward" was a joke, obviously

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Will Deacon]

On Tue, May 05, 2020 at 12:05:19PM +0100, Dave Martin wrote:
> On Tue, May 05, 2020 at 11:44:55AM +0100, Will Deacon wrote:
> > Hi Dave,
> > 
> > On Mon, May 04, 2020 at 04:32:35PM +0100, Dave Martin wrote:
> > > I considering trying to plug some gaps in the arch-specific ABI
> > > documentation in the linux man-pages, specifically for arm64 (and
> > > possibly arm, where compat means we have some overlap).
> > > 
> > > For arm64, there are now significant new extensions (Pointer
> > > authentication, SVE, MTE etc.)  Currently there is some user-facing
> > > documentation mixed in with the kernel-facing documentation in the
> > > kernel tree, but this situation isn't ideal.
> > > 
> > > Do you have an opinion on where in the man-pages documentation should be
> > > added, and how to structure it?
> > > 
> > > 
> > > Affected areas include:
> > > 
> > >  * exec interface
> > >  * aux vector, hwcaps
> > >  * arch-specific signals
> > >  * signal frame
> > >  * mmap/mprotect extensions
> > >  * prctl calls
> > >  * ptrace quirks and extensions
> > >  * coredump contents
> > > 
> > > 
> > > Not everything has an obvious home in an existing page, and adding
> > > specifics for every architecture could make some existing manpages very
> > > unwieldy.
> > > 
> > > I think for some arch features, we really need some "overview" pages
> > > too: just documenting the low-level details is of limited value
> > > without some guide as to how to use them together.
> > > 
> > > 
> > > Does the following sketch look reasonable?
> > > 
> > >  * man7/arm64.7: new page: overview of arm64-specific ABI extensions
> > > 
> > >  * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
> > >    overview of arm64 SVE ABI
> > > 
> > >  * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
> > >    arm64 ptrace extensions
> > 
> > Michael has been nagging me on and off about that for, what, 10 years now?
> > I would therefore be very much in favour of having our ptrace extensions
> > documented!
> > 
> > We could even put this stuff under Documentation/arm64/man/ if it's deemed
> > too CPU-specific for the man-pages project, but my preference would still
> > be for it to be hosted there alongside all the other man pages.
> 
> Heh, perhaps we could build that into the kernel and mount it somewhere.

Argh, don't joke too loudly about things like that!

> Seriously though,
> 
> I guess I can start off with straightforward small things for which the
> documentation has an obvious home (like prctls[*]) and then move on to
> the bigger stuff like ptrace.
> 
> If people start shouting about a page getting too big or messy I can try
> to split it up.
> 
> Make sense?

Sure, works for me.

Will

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Michael Kerrisk (man-pages)]

> If people start shouting about a page getting too big or messy I can try
> to split it up.
> 
> Make sense?

Yes.

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Russell King - ARM Linux admin]

On Tue, May 05, 2020 at 11:44:55AM +0100, Will Deacon wrote:
> Michael has been nagging me on and off about that for, what, 10 years now?
> I would therefore be very much in favour of having our ptrace extensions
> documented!
> 
> We could even put this stuff under Documentation/arm64/man/ if it's deemed
> too CPU-specific for the man-pages project, but my preference would still
> be for it to be hosted there alongside all the other man pages.

Stuffing random things into the kernel tree is painful for some people.

For example, if you cross-build your kernel, then the stuff in the
tools/ subdirectory is totally useless (I think everything except
perf) because you can't build it.

Let's stop making the mistake of constantly shoving stuff into the
kernel source tree.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Will Deacon]

On Tue, May 05, 2020 at 01:43:51PM +0100, Russell King - ARM Linux admin wrote:
> On Tue, May 05, 2020 at 11:44:55AM +0100, Will Deacon wrote:
> > Michael has been nagging me on and off about that for, what, 10 years now?
> > I would therefore be very much in favour of having our ptrace extensions
> > documented!
> > 
> > We could even put this stuff under Documentation/arm64/man/ if it's deemed
> > too CPU-specific for the man-pages project, but my preference would still
> > be for it to be hosted there alongside all the other man pages.
> 
> Stuffing random things into the kernel tree is painful for some people.
> 
> For example, if you cross-build your kernel, then the stuff in the
> tools/ subdirectory is totally useless (I think everything except
> perf) because you can't build it.
> 
> Let's stop making the mistake of constantly shoving stuff into the
> kernel source tree.

For userspace tools, I'm inclined to agree, but this is just documentation
so it shouldn't cause any issues with cross building. But to be clear: I'd
still prefer it to be part of the man-pages project, and would only consider
it for inclusion in the kernel tree if it was rejected for being too
CPU-specific.

Will

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Russell King - ARM Linux admin]

On Tue, May 05, 2020 at 02:06:30PM +0100, Will Deacon wrote:
> On Tue, May 05, 2020 at 01:43:51PM +0100, Russell King - ARM Linux admin wrote:
> > On Tue, May 05, 2020 at 11:44:55AM +0100, Will Deacon wrote:
> > > Michael has been nagging me on and off about that for, what, 10 years now?
> > > I would therefore be very much in favour of having our ptrace extensions
> > > documented!
> > > 
> > > We could even put this stuff under Documentation/arm64/man/ if it's deemed
> > > too CPU-specific for the man-pages project, but my preference would still
> > > be for it to be hosted there alongside all the other man pages.
> > 
> > Stuffing random things into the kernel tree is painful for some people.
> > 
> > For example, if you cross-build your kernel, then the stuff in the
> > tools/ subdirectory is totally useless (I think everything except
> > perf) because you can't build it.
> > 
> > Let's stop making the mistake of constantly shoving stuff into the
> > kernel source tree.
> 
> For userspace tools, I'm inclined to agree, but this is just documentation
> so it shouldn't cause any issues with cross building. But to be clear: I'd
> still prefer it to be part of the man-pages project, and would only consider
> it for inclusion in the kernel tree if it was rejected for being too
> CPU-specific.

I don't think that should be a concern; the man-pages project already
contains documentation that is specific to kernel versions, including
documentation for interfaces that are architecture specific (such as
prctl is a big one, ptrace to a lesser extent.)  syscall(2) contains
a whole bunch of architecture stuff about the calling convention for
syscalls.

Interestingly, I notice that syscall(2) is wrong for arm/OABI. I am
not surprised, because that documentation never came my way, and I am
the author of the kernel's OABI syscall interface.

It claims:

       arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
       ──────────────────────────────────────────────────────────────
       arm/OABI      a1    a2    a3    a4    v1    v2    v3

whereas, at the time I invented it, I decided that it shall pass
arguments in r0 to r6.  That's r0 to r6.  Not the APCS register
names that this document claims.  Not everything in OABI is APCS,
as illustrated here - APCS passes the first four arguments in a1
to a4, and then the rest on the stack.  The OABI syscall interface
doesn't do that.

I guess that's what happens when someone else writes interface
documentation and doesn't bother to get it reviewed by those who
created the interface in the first place.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Michael Kerrisk (man-pages)]

On 5/5/20 2:43 PM, Russell King - ARM Linux admin wrote:
> On Tue, May 05, 2020 at 11:44:55AM +0100, Will Deacon wrote:
>> Michael has been nagging me on and off about that for, what, 10 years now?
>> I would therefore be very much in favour of having our ptrace extensions
>> documented!
>>
>> We could even put this stuff under Documentation/arm64/man/ if it's deemed
>> too CPU-specific for the man-pages project, but my preference would still
>> be for it to be hosted there alongside all the other man pages.
> 
> Stuffing random things into the kernel tree is painful for some people.

Yes, and too often not easily noticeable for user-space programmers.
> 
> For example, if you cross-build your kernel, then the stuff in the
> tools/ subdirectory is totally useless (I think everything except
> perf) because you can't build it.
> 
> Let's stop making the mistake of constantly shoving stuff into the
> kernel source tree.

Agrred.

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Michael Kerrisk (man-pages)]

>>  * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
>>    arm64 ptrace extensions
> 
> Michael has been nagging me on and off about that for, what, 10 years now?

Perhaps not that long, but a while now ;-).

> I would therefore be very much in favour of having our ptrace extensions
> documented!
> 
> We could even put this stuff under Documentation/arm64/man/ if it's deemed
> too CPU-specific for the man-pages project, but my preference would still
> be for it to be hosted there alongside all the other man pages.

Agreed; manual pages is I think a better place.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Michael Kerrisk (man-pages)]

Hello Dave, et al.

On 5/4/20 5:32 PM, Dave Martin wrote:
> Hi all,
> 
> I considering trying to plug some gaps in the arch-specific ABI
> documentation in the linux man-pages, specifically for arm64 (and
> possibly arm, where compat means we have some overlap).

Sounds good to me.

> For arm64, there are now significant new extensions (Pointer
> authentication, SVE, MTE etc.)  Currently there is some user-facing
> documentation mixed in with the kernel-facing documentation in the
> kernel tree, but this situation isn't ideal.
> 
> Do you have an opinion on where in the man-pages documentation should be
> added, and how to structure it?
> 
> 
> Affected areas include:
> 
>  * exec interface

Not sure what the details are here, so I have no opinion yet.
But probably, as additions to execve(2).

>  * aux vector, hwcaps

==> getauxval(3)

>  * arch-specific signals
>  * signal frame

Not sure what the details are here, so I have no opinion yet.

>  * mmap/mprotect extensions

See below.

>  * prctl calls

As additions in prctl(2) would be fine, I think.

>  * ptrace quirks and extensions

See below.

>  * coredump contents

Not sure what the details are here, so I have no opinion yet.
Possibly as additions to core(5).

> Not everything has an obvious home in an existing page, 

Yes.

> and adding
> specifics for every architecture could make some existing manpages very
> unwieldy.

Still, I think it's worth adding details, especially for widely
used architectures.
 
> I think for some arch features, we really need some "overview" pages
> too: just documenting the low-level details is of limited value
> without some guide as to how to use them together.
> 
> 
> Does the following sketch look reasonable?
> 
>  * man7/arm64.7: new page: overview of arm64-specific ABI extensions

I'm a little unclear on what would land in here, but sounds 
reasonable in principle.

>  * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
>    overview of arm64 SVE ABI

Sounds reasonable to me.

>  * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
>    arm64 ptrace extensions

I think maybe better is: ptrace-arm64.2

I'm agnostic about whether there should be a new page, or whether 
these should be added to ptrace(2). But, we could start with the
idea of a new page.

>  * man2/mmap.2: extend with arm64-specific flags (only two flags, so we
>    add them to the existing man page rather than creating a new one).

Sounds good to me

> etc.
> 
> 
> Ideally, I'd like to adopt a pattern that other arches can follow.

Well, if they do follow. Arch-specific documentation is woefully
thin at the moment. I'm not going to worry too much about the right
pattern (don't let perfect get in the way of good, yadda, yadda),
until I get so much arch-specific documentation that some refactoring
may be required. (I'm not going to hold my breath waiting for that
to happen ;-).)

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: RFC: Adding arch-specific user ABI documentation in linux-man

[Dave Martin]

On Wed, May 06, 2020 at 12:47:17PM +0200, Michael Kerrisk (man-pages) wrote:
> Hello Dave, et al.
> 
> On 5/4/20 5:32 PM, Dave Martin wrote:
> > Hi all,
> > 
> > I considering trying to plug some gaps in the arch-specific ABI
> > documentation in the linux man-pages, specifically for arm64 (and
> > possibly arm, where compat means we have some overlap).
> 
> Sounds good to me.
> 
> > For arm64, there are now significant new extensions (Pointer
> > authentication, SVE, MTE etc.)  Currently there is some user-facing
> > documentation mixed in with the kernel-facing documentation in the
> > kernel tree, but this situation isn't ideal.
> > 
> > Do you have an opinion on where in the man-pages documentation should be
> > added, and how to structure it?
> > 
> > 
> > Affected areas include:
> > 
> >  * exec interface
> 
> Not sure what the details are here, so I have no opinion yet.
> But probably, as additions to execve(2).

Having looked at that page again, probably yes.

This would include things like how arch-specific registers get reset,
and how various thread properties are reset/inherited.

Possibly, much of this would be documented by cross-referencing other
pages.

> 
> >  * aux vector, hwcaps
> 
> ==> getauxval(3)

Ah, yes.  I keep forgetting that because it's provided by libc.

There are a few things missing, but we can add them in there.

> >  * arch-specific signals
> >  * signal frame
> 
> Not sure what the details are here, so I have no opinion yet.

Stuff like architecture-specific si_codes and how they are used.

Signal handlers that poke about in the signal frame need to know how it
is structured, and for arm64 this is now quite complex.  I want to
document some example code for parsing it somewhere, and I feel that
will be too much noise for signal(7).

I guess I can try to write something and than we can figure out where to
put it.

> 
> >  * mmap/mprotect extensions
> 
> See below.
> 
> >  * prctl calls
> 
> As additions in prctl(2) would be fine, I think.
> 
> >  * ptrace quirks and extensions
> 
> See below.
> 
> >  * coredump contents
> 
> Not sure what the details are here, so I have no opinion yet.
> Possibly as additions to core(5).

Things like arch-specific regsets.  Some of that can probabably live
in the ptrace page, with cross-references from core(5).

> > Not everything has an obvious home in an existing page, 
> 
> Yes.
> 
> > and adding
> > specifics for every architecture could make some existing manpages very
> > unwieldy.
> 
> Still, I think it's worth adding details, especially for widely
> used architectures.

Agreed.  If there's a lot of explanation required for somethings which
feels out of scope for the core page, then I might still be tempted to
push some of that out into a more arch-specific page and cross-reference
it.  We can play that by ear, though.

> > I think for some arch features, we really need some "overview" pages
> > too: just documenting the low-level details is of limited value
> > without some guide as to how to use them together.
> > 
> > 
> > Does the following sketch look reasonable?
> > 
> >  * man7/arm64.7: new page: overview of arm64-specific ABI extensions
> 
> I'm a little unclear on what would land in here, but sounds 
> reasonable in principle.

I probably won't attempt to add such a page until I find a use for it.

I mainly intended this as an overview of what areas are impacted by the
architecture and which other pages to go look at.

> >  * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
> >    overview of arm64 SVE ABI
> 
> Sounds reasonable to me.
> 
> >  * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
> >    arm64 ptrace extensions
> 
> I think maybe better is: ptrace-arm64.2

Agreed.  That's probably easier to for people to place in their mental
map.

> 
> I'm agnostic about whether there should be a new page, or whether 
> these should be added to ptrace(2). But, we could start with the
> idea of a new page.
> 
> >  * man2/mmap.2: extend with arm64-specific flags (only two flags, so we
> >    add them to the existing man page rather than creating a new one).
> 
> Sounds good to me
> 
> > etc.
> > 
> > 
> > Ideally, I'd like to adopt a pattern that other arches can follow.
> 
> Well, if they do follow. Arch-specific documentation is woefully
> thin at the moment. I'm not going to worry too much about the right
> pattern (don't let perfect get in the way of good, yadda, yadda),
> until I get so much arch-specific documentation that some refactoring
> may be required. (I'm not going to hold my breath waiting for that
> to happen ;-).)

Sure.  I don't want to do something obviously broken, but it sounds like
you're reasonably happy with my approach.  In any case I'll start with the
simple stuff.

Where it's easy to do so, I may try to trawl up some undocumented non-
ARM stuff, but I'm not the expert there...

Cheers
---Dave