Date:	Wed, 25 May 2011 10:35:59 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Valdis.Kletnieks@...edu,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>,
	netdev@...r.kernel.org
Subject: Re: [GIT PULL] Namespace file descriptors for 2.6.40

On Wed, May 25, 2011 at 10:25, Ingo Molnar <mingo@...e.hu> wrote:
> * Valdis.Kletnieks@...edu <Valdis.Kletnieks@...edu> wrote:
>
>> On Tue, 24 May 2011 09:16:28 +0200, Ingo Molnar said:
>> > * Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> > > My gut feel says we should really implement an
>> > > include/asm-generic/unistd-common.h to include all new system calls.
>> > >
>> > > That way there would be only one file to touch instead of 50. Certainly it
>> > > works for include/asm-generic/unistd.h for the architectures that use it.
>> > > And all we really need is just a little abstraction on that concept.
>> >
>> > I suppose that could be tried, although in practice it would probably be
>> > somewhat complex due to the various compat syscall handling differences.
>>
>> Can somebody fill us newcomers in on the arch-aeology of why some syscalls have
>> different numbers on different archs? I know it's partially because some simply
>> didn't implement some syscalls so there were numbering mismatches, but would it
>> have been *that* hard to wire all of those skipped syscalls up to one stub
>> 'return -ENOSYS'?
>
> It was done so for hysterical raisons mostly, and once a bad ABI is done it's
> very hard to undo it: beyond pushing the 'good ABI' you'd also still have to
> deal with the bad ABI for a decade or more.
>
> So the background is that most architectures start out as quick concept
> prototypes, doing:
>
>        cp -a arch/existingarch arch/newarch
>
> where 'existingarch' used to be arch/i386/ in the early days. Now i386 had a
> fair amount of x86 specific syscalls that were naturally removed from
> 'newarch'. Those created 'holes' in the numbers, which were then filled in with
> new syscalls - a nice idea in itself!
>
> Also sometimes 'newarch' did a 'clean', compressed list of syscall numbers
> straight away, reordering syscalls. Once the 'quick prototype' hack starts
> working on real hardware, once the syscall numbers get into the C library and
> binutils it's very hard to ever transition away: you'd break the world!
>
> An added source of noise that architectures tend to add new syscalls in a
> different order: some are more interesting to them - some less.
>
> So these syscall table hacks done very early during an arch's lifetime stick
> around and create wild numbering noise in 20+ syscall tables:
>
>                                       [ slightly edited for readability ]
>
>  arch/alpha/include/asm/unistd.h:      #define __NR_perf_event_open 493
>  arch/arm/include/asm/unistd.h:        #define __NR_perf_event_open 364
>  arch/blackfin/include/asm/unistd.h:   #define __NR_perf_event_open 369
>  arch/frv/include/asm/unistd.h:        #define __NR_perf_event_open 336
>  arch/m68k/include/asm/unistd.h:       #define __NR_perf_event_open 332
>  arch/microblaze/include/asm/unistd.h: #define __NR_perf_event_open 366
>  arch/mips/include/asm/unistd.h:       #define __NR_perf_event_open 333
>  arch/mips/include/asm/unistd.h:       #define __NR_perf_event_open 292
>  arch/mips/include/asm/unistd.h:       #define __NR_perf_event_open 296
>  arch/mn10300/include/asm/unistd.h:    #define __NR_perf_event_open 337
>  arch/parisc/include/asm/unistd.h:     #define __NR_perf_event_open 318
>  arch/powerpc/include/asm/unistd.h:    #define __NR_perf_event_open 319
>  arch/s390/include/asm/unistd.h:       #define __NR_perf_event_open 331
>  arch/sh/include/asm/unistd_32.h:      #define __NR_perf_event_open 336
>  arch/sh/include/asm/unistd_64.h:      #define __NR_perf_event_open 364
>  arch/sparc/include/asm/unistd.h:      #define __NR_perf_event_open 327
>  arch/x86/include/asm/unistd_32.h:     #define __NR_perf_event_open 336
>  arch/x86/include/asm/unistd_64.h:     #define __NR_perf_event_open 298
>
> To fix this we'd create a new, clean offset defined by each architecture, and a
> generic enumeration of new syscalls.
>
> This would make it much easier to add new, generic syscalls to all
> architectures indeed.
>
> It would still leave compat syscall wrappers unaddressed though: those are
> often numbered differently and sometimes need arch specific wrapper entry
> functions, which then call the real generic syscall.
>
> But at least the primary, 'native' syscall table of every arch could be kept
> rather fresh via generic enumeration.

So we can start all over at offset 501 (alpha just started using 500)
with a unified,
clean, and compressed list of syscalls? Or do we have some more other-os-compat
syscalls around in this range?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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