On Wed, Sep 05, 2012 at 08:13:12PM +0100, Russell King - ARM Linux wrote: > On Wed, Aug 15, 2012 at 02:49:54PM +0000, Arnd Bergmann wrote: > > It's fairly unusual to have out of line get_user/put_user functions. > > What is the reason for this, other than copying from ARM? > > Actually, we never used to out of line on ARM, and then I experimented, > and found there was a net benefit - not only in code size but also > there appeared to be a performance benefit by out of lining them. 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. 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. For now I changed the arm64 code to inline as most of the other architectures but I'll revisit this when I get some hardware. Another thing to be optimised is the size of the exception table entry. I use two .quad declarations for the fault address and branch but I guess they could be made relative to PAGE_OFFSET and only use .long (or some prel31). -- 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
