linux-kernel - Re: asm-generic/unistd.h and glibc use of NR

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Message-Id: <201009282208.52075.arnd@arndb.de>
Date:	Tue, 28 Sep 2010 22:08:51 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Joseph S. Myers" <joseph@...esourcery.com>
Cc:	Chris Metcalf <cmetcalf@...era.com>, linasvepstas@...il.com,
	linux-kernel@...r.kernel.org, libc-ports@...rceware.org,
	andrew@...esourcery.com
Subject: Re: asm-generic/unistd.h and glibc use of NR_ipc

On Tuesday 28 September 2010 18:41:41 Joseph S. Myers wrote:
> In my experience very little of a glibc port relates to the particular set 
> of syscalls present on a particular architecture (and what does often 
> consists of one-line files that #include the file for i386 or some other 
> convenient architecture); changing a port from an old syscall ABI to the 
> generic one would mean making it use the new set of generic files in 
> preference to some existing sysdeps/unix/sysv/linux files, and maybe 
> having fewer bits/ headers if some of the architecture differences there 
> are avoided, but no real differences to any of the bits that aren't purely 
> mechanical such as the implementation of each architecture's TLS ABI, 
> other pieces of the ELF ABI, the set of atomic operations available, 
> startup code, longjmp, profiling, clone, the interface used to call an 
> individual syscall, etc. - remember that you can't build any vaguely 
> recent glibc without full shared library and NPTL support.
> 
> A lot of the changes made to maintain an existing port relate to 
> constants in the duplicated bits/* headers which it might be good to 
> duplicate less if future architectures share values for all the constants, 
> but that's not where most of the complexity in the port lies.  MIPS is 
> probably the worst case for code dealing with syscall ABI peculiarities 
> (given the three different ABIs, and n64 having different userspace stat 
> and stat64 structures), but even there they aren't much of a source of 
> complexity (MIPS ELF peculiarities are much more an issue).

Good point. As a kernel developer I tend to see glibc too much as
just a front-end for the system calls, but of course you are right
that it is much more that requires a lot of hard work for an architecture
port.
I did not mean to discredit the good work that has gone into the nios2
port or any part of upstream glibc.

The question that Linas was asking is specifically about the syscall
ABI and this is what I'm mostly interested in as well. A significant
part of the maintainance work in the kernel is just keeping up with
syscall ABI changes, which now gets minimized for new architectures.

	Arnd
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