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: > > 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? > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
