Messages in this thread | | | Date | Wed, 5 Sep 2012 22:05:34 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH v2 20/31] arm64: User access library function |
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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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