[PATCH v2 20/31] arm64: User access library function

On Wed, Sep 05, 2012 at 10:05:34PM +0100, Russell King - ARM Linux wrote:

On Wed, Sep 05, 2012 at 10:01:37PM +0100, Catalin Marinas wrote:

quoted

There are indeed a few KB gain in code size but that's probably coming
from the exception table since otherwise you just replace a bl with
ldrt. It depends on what the compiler does as well, the arm code has
some carefully chosen registers when calling the __get_user_x function.

It's more than that - it's not just the ldr but also a zeroing of a
temporary register to hold the error code should the instruction fault.
So it's not only the exception tables but also an increase in the
main path - and that's where you benefit from having it out of line and
thereby a hotter i-cache.

On 32-bit we have __get_user() inline and get_user() out of line. What
was the history behind this?

quoted

If you do the access_ok inline and the __get_user_x separately, the size
increase is even greater (at least in the arm64 case it can get to over
20KB). I think x86 does the access_ok check out of line.

Please talk to Will about get_user() and put_user().  Afterwards you
will definitely want to keep them out of line on 64-bit ARM.

As I said, I already made the change to always inline get_user/put_user
with some penalty in the Image size but it makes the code cleaner. I'm
not entirely convinced of the performance gain/loss especially on ARMv8
cores with physically tagged caches. There is room for optimisation when
I get real silicon.

-- 
Catalin