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    SubjectRe: [PATCH] ARM: alignment: setup alignment handler earlier
    On Wed, Sep 07, 2011 at 03:40:16PM +0100, Catalin Marinas wrote:
    > On Wed, Sep 07, 2011 at 02:35:04PM +0100, John Ogness wrote:
    > > From 6f3f381800367127dc6430d9b9fa9bd6fc6d8ed0 Mon Sep 17 00:00:00 2001
    > > From: John Ogness <>
    > >
    > > The alignment exception handler is setup fairly late in
    > > the boot process (fs_initcall). However, with newer gcc
    > > versions and the compiler option -fconserve-stack, code
    > > accessing unaligned data is generated in functions that
    > > are called earlier, for example pcpu_dump_alloc_info().
    > > This results in unhandled alignment exceptions during
    > > boot. By setting up the exception handler sooner, we
    > > reduce the window where a compiler may generate code
    > > accessing unaligned data.
    > While this reduces the window and fixes this particular case, it still
    > doesn't solve the problem. We never know when some unaligned access
    > would be generated for some earlier code.

    Is the problem even solvable? There are instructions on ARMv6+ which
    always produce an alignment fault (eg, ldrd) irrespective of strict
    alignment checking.

    There will always be a window for those - when we don't have the
    vectors setup - where we potentically could take such a fault and end
    up crashing. So I'm not sure that the right answer is what's being

    What it's saying to me is that building the kernel in a way that gcc
    intentionally generates misaligned accesses _will_ bite us in random
    unknown ways sooner or later.

    I don't think its feasible to build some of the kernel with alignment
    faults enabled and other parts with it disabled - that's going to be
    very fragile and probably be hell to identify which parts should and
    should not.

    So I think where we're heading is to need gcc _not_ to create any code
    what so ever which would create a misalignment fault.

     \ /
      Last update: 2011-09-07 18:31    [W:0.022 / U:91.204 seconds]
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