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

Russell King - ARM Linux linux at arm.linux.org.uk
Wed Sep 5 17:05:34 EDT 2012

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

> 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.

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