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Subject: Re: [PATCH RESEND 1/9] crypto: caam/jr - add fallback for XTS with
 more than 8B IV
To:     Ard Biesheuvel <ardb@kernel.org>
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On 9/15/2020 1:26 PM, Ard Biesheuvel wrote:
> On Tue, 15 Sep 2020 at 13:02, Horia Geantă <horia.geanta@nxp.com> wrote:
>>
>> On 9/14/2020 9:20 PM, Ard Biesheuvel wrote:
>>> On Mon, 14 Sep 2020 at 20:12, Horia Geantă <horia.geanta@nxp.com> wrote:
>>>>
>>>> On 9/14/2020 7:28 PM, Ard Biesheuvel wrote:
>>>>> On Mon, 14 Sep 2020 at 19:24, Horia Geantă <horia.geanta@nxp.com> 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