x86: rewrite '__copy_user_nocache' function

I didn't really want to do this, but as part of all the other changes to
the user copy loops, I've been looking at this horror.

I tried to clean it up multiple times, but every time I just found more
problems, and the way it's written, it's just too hard to fix them.

For example, the code is written to do quad-word alignment, and will use
regular byte accesses to get to that point.  That's fairly simple, but
it means that any initial 8-byte alignment will be done with cached
copies.

However, the code then is very careful to do any 4-byte _tail_ accesses
using an uncached 4-byte write, and that was claimed to be relevant in
commit a82eee742452 ("x86/uaccess/64: Handle the caching of 4-byte
nocache copies properly in __copy_user_nocache()").

So if you do a 4-byte copy using that function, it carefully uses a
4-byte 'movnti' for the destination.  But if you were to do a 12-byte
copy that is 4-byte aligned, it would _not_ do a 4-byte 'movnti'
followed by a 8-byte 'movnti' to keep it all uncached.

Instead, it would align the destination to 8 bytes using a
byte-at-a-time loop, and then do a 8-byte 'movnti' for the final 8
bytes.

The main caller that cares is __copy_user_flushcache(), which knows
about this insanity, and has odd cases for it all.  But I just can't
deal with looking at this kind of "it does one case right, and another
related case entirely wrong".

And the code really wasn't fixable without hard drugs, which I try to
avoid.

So instead, rewrite it in a form that hopefully not only gets this
right, but is a bit more maintainable.  Knock wood.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 4f1a40a86534..01932af64193 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -71,6 +71,6 @@ ifneq ($(CONFIG_GENERIC_CSUM),y)
 endif
         lib-y += clear_page_64.o copy_page_64.o
         lib-y += memmove_64.o memset_64.o
-        lib-y += copy_user_64.o
+        lib-y += copy_user_64.o copy_user_uncached_64.o
 	lib-y += cmpxchg16b_emu.o
 endif