linux-kernel - Re: [PATCH] reduce export symbol CRC table size on 64-bit archs

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Date:	Fri, 03 Jul 2009 22:16:28 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Ingo Molnar <mingo@...e.hu>, tony.luck@...el.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Rusty Russell <rusty@...tcorp.com.au>, hpa@...or.com,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-modules@...r.kernel.org
Subject: Re: [PATCH] reduce export symbol CRC table size on 64-bit archs

Jan Beulich napsal(a):
> Since these CRCs are really only 32-bit quantities, there's no need to
> store them in 64-bit slots. Since, however, gcc doesn't allow
> respective initializations, asm() constructs get used to create the CRC
> tables (and its for that reason that the patch only makes x86-64 and
> ia64 utilize that functionality, as I can't verify this doesn't break
> in some subtle way elsewhere).
> 
...
>  struct modversion_info
>  {
> -	unsigned long crc;
> +	ksym_crc_t crc;
>  	char name[MODULE_NAME_LEN];
>  };

This change breaks module-init-tools:
Before:
$ /sbin/modprobe --dump-modversions _build/drivers/usb/core/usbcore.ko
| head
0xb49b735a      module_layout
0xdb7e6a70      bus_register
0xe3a25bcc      device_remove_file
0x1fedf0f4      __request_region
0x37429b3b      device_remove_bin_file
0x27ac237c      cdev_del
0xf3be63b3      per_cpu__current_task
0x26b76a45      kmalloc_caches
0x5a34a45c      __kmalloc
0x1c820358      cdev_init

After:
$ /sbin/modprobe --dump-modversions
_build-crc-int/drivers/usb/core/usbcore.ko  | head
0x75646f6d91ea7b5c      le_layout
0x5f7375623e215f43      register
0x69766564848c67e2      ce_remove_file
0x65725f5f1fedf0f4      quest_region
0x69766564a0c29c25      ce_remove_bin_file
0x766564633ce98bc2      _del
0x5f72657012c9615b      cpu__current_task
0x6c616d6b6ea6c464      loc_caches
0x6d6b5f5f5a34a45c      alloc
0x766564631e5d885b      _init

It also breaks the newly added depmod -E option (check symbol versions),
which also reads the struct modversion_info array (*). Is it possible
name the section differently (__versions2?) on those architectures where
the size changes, so that it is possible to fix m-i-t in a
backwards-compatible manner?

Michal


(*) dump_modvers() and load_symbols() in
http://git.kernel.org/?p=utils/kernel/module-init-tools/module-init-tools.git;a=blob;f=elfops_core.c;hb=HEAD
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