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. 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. Would it be ok to use: #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && !defined(CONFIG_ARM) to workaround the ARMv7 inconsistency? Thanks, Horia
