Re: [PATCH] Ensure PSR.ac is cleared for early userspace

["David Mosberger-Tang" <dmosberger@gmail.com>]

Tony,

Have you checked the IA64 ABI?  I remember there being a section
talking about the state in which signal handlers should be started.  I
don't remember off hand if it said anything about psr.ac.

  --david

On 11/14/08, Luck, Tony <tony.luck@intel.com> wrote:
>  GLOBAL_ENTRY(kernel_execve)
>  +       rum IA64_PSR_AC
>         mov r15=__NR_execve                     // put syscall number in place
>         break __BREAK_SYSCALL
>         br.ret.sptk.many rp
>
>  I'm not overly happy with this because I still don't understand how
>  the PSR.ac bit became set in this context.  This patch papers
>  over the problem by clearing psr.ac ... but it shouldn't be
>  set at this point!
>
>  As far as I can see usage of psr.ac is as follows:
>
>  1) When Linux first boots the code in head.S transitions to
>  virtual mode ... this code sets PSR.ac=0.
>
>  2) Eight of the code paths in ivt.S set PSR.ac=1 before
>  transitioning to C code.  Thus interrupt handlers, page faults
>  and other traps are executed with .ac=1.  This is controlled by the
>  PSR_DEFAULT_BITS define ... which is (and I think always has)
>  been defined to set psr.ac.
>
>  [I'm a bit puzzled as to why we want strict alignment checking
>  in interrupt/fault handlers, but are satisfied with more relaxed
>  behaivour in base code]
>
>  I'm also puzzled why this issue only just surfaced, and why I
>  only see it on generic kernels.
>
>  The problem may be related to some interrupt behavior.  If I
>  tweak just the "__interrupt" code to not set psr.ac (but leave
>  the other seven fault handlers setting psr.ac=1) then I don't
>  see any unaligned traps in early init code.
>
>  -Tony
>
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