On 9/15/2020 1:26 PM, Ard Biesheuvel wrote: > On Tue, 15 Sep 2020 at 13:02, Horia Geantă <horia.geanta@xxxxxxx> wrote: >> >> On 9/14/2020 9:20 PM, Ard Biesheuvel wrote: >>> On Mon, 14 Sep 2020 at 20:12, Horia Geantă <horia.geanta@xxxxxxx> wrote: >>>> >>>> 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. >>>> >>> >>> But the minimal optimization that is possible here (one ldrd/ldm >>> instruction vs two ldr instructions) is defeated by the fact that you >>> are using a conditional branch to select between the two. And this is >>> not even a hot path to begin with, >>> >> This is actually on the hot path (encrypt/decrypt callbacks), >> but you're probably right that the conditional branching is going to offset >> the optimized code. >> > > This is called once per XTS request, right? And you are saying the > extra cycle makes a difference? > Yes, once per request and no, not super-important. >> To avoid branching, code could be rewritten as: >> >> #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS >> size = *(u64 *)(req->iv + (ivsize / 2)); >> #else >> size = get_unaligned((u64 *)(req->iv + (ivsize / 2))); >> #endif >> >> however in this case ARMv7 would suffer since >> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y and >> ldrd/ldm for accesses not word-aligned are inefficient - lead to traps. >> > > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS means 'just use the unaligned > accessors as they are basically free'. Casting a potentially > misaligned u8* to a u64* is not permitted by the C standard. > Seems that I misunderstood CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS. Looking at its usage, e.g. ether_addr_equal() or __crypto_memneq_*(), I see similar casts of pointers possibly misaligned. >> Would it be ok to use: >> #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && !defined(CONFIG_ARM) >> to workaround the ARMv7 inconsistency? >> > > No, please just use the get_unaligned() accessor. > Ok. Thanks, Horia
