Re: [patch 2.6.11] __copy_user breaks on unaligned src

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Fri, 18 Mar 2005 18:04:37 +1100, Keith Owens <kaos@sgi.com> said:

  Keith> memcpy_mck.S::__copy_user breaks in the prefetch code under these
  Keith> conditions :-

  Keith> * src is unaligned and
  Keith> * dst is near the end of a page and
  Keith> * the page after dst is unmapped.

  Keith> The loop count in r21 is 1 value too high.  A length of 0x100 gives
  Keith> ar.lc = r21 = 2.  .unaligned_src incorrectly copies r21 into ar.lc,
  Keith> when it should copy cnt, so the lfetch lines are executed 3 times, not
  Keith> 2.  That takes dst_pre_mem past the end of the page and into an
  Keith> unallocated area, oops.

That's a good thing to fix (it's definitely a performance bug).  However,
lfetch.fault should be safe to use even on unmapped memory.  See this
code in ia64_do_page_fault():

 /*
  * This fault was due to a speculative load or lfetch.fault, set the "ed"
  * bit in the psr to ensure forward progress.  (Target register will get a
  * NaT for ld.s, lfetch will be canceled.)
  */

I don't see off-hand why this wouldn't work as intended.

	--david