The DR7 register on i386/x86_64 has a special meaning, so there is a
special check to do.  Since the code is rather difficult, I added an
explaination about it.  Also, while studying the i386 Intel Manual, I saw
that x86_64, even on 32bit emulation, allows using values which are
disallowed on i386.  It's almost obvious that what it allows is setting a
8-byte wide data watchpoint (in fact I double checked the AMD manuals, just
in case, and this is true; I couldn't find a mention of this in my Intel
manuals).

But since the original ia32 emulation code has this comment:

	/* You are not expected to understand this ... I don't neither. */

I am dubious that the code in ptrace32.c is wrong: does x86_64 supports
8byte-wide watchpoints in 32-bit emulation?  I've checked the AMD manual
Vol.  2 (no.  24593), which says that to set the length to 8 bits "long
mode must be enabled".  This means, actually, that the current code is
safe, notwithstanding the comment (which I removed).
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Cc: Roland McGrath <roland@redhat.com>, Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>