On 9/14/2020 7:28 PM, Ard Biesheuvel wrote: > On Mon, 14 Sep 2020 at 19:24, Horia Geantă <horia.geanta@xxxxxxx> wrote: >> >> On 9/9/2020 1:10 AM, Herbert Xu wrote: >>> On Tue, Sep 08, 2020 at 01:35:04PM +0300, Horia Geantă wrote: >>>> >>>>> Just go with the get_unaligned unconditionally. >>>> >>>> Won't this lead to sub-optimal code for ARMv7 >>>> in case the IV is aligned? >>> >>> If this should be optimised in ARMv7 then that should be done >>> in get_unaligned itself and not open-coded. >>> >> I am not sure what's wrong with avoiding using the unaligned accessors >> in case data is aligned. >> >> Documentation/core-api/unaligned-memory-access.rst clearly states: >> These macros work for memory accesses of any length (not just 32 bits as >> in the examples above). Be aware that when compared to standard access of >> aligned memory, using these macros to access unaligned memory can be costly in >> terms of performance. >> >> So IMO it makes sense to use get_unaligned() only when needed. >> There are several cases of users doing this, e.g. siphash. >> > > For ARMv7 code, using the unaligned accessors unconditionally is fine, > and it will not affect performance. > > In general, when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is defined, > you can use the unaligned accessors. If it is not, it helps to have > different code paths. > arch/arm/include/asm/unaligned.h doesn't make use of linux/unaligned/access_ok.h, even if CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set. I understand the comment in the file, however using get_unaligned() unconditionally takes away the opportunity to generate optimized code (using ldrd/ldm) when data is aligned. > This is a bit murky, and through the years, the interpretation of > unaligned-memory-access.rst has shifted a bit, but in this case, it > makes no sense to make the distinction. > Thanks, Horia
