linux-kernel - Re: [PATCH RESEND 1/9] crypto: caam/jr - add fallback for XTS with more than 8B IV

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Message-ID: <4393bf96-30fd-0d1c-73fe-f5ef7c967f76@nxp.com>
Date:   Tue, 15 Sep 2020 13:02:41 +0300
From:   Horia Geantă <horia.geanta@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Andrei Botila (OSS)" <andrei.botila@....nxp.com>,
        Aymen Sghaier <aymen.sghaier@....com>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND 1/9] crypto: caam/jr - add fallback for XTS with
 more than 8B IV

On 9/14/2020 9:20 PM, Ard Biesheuvel wrote:
> On Mon, 14 Sep 2020 at 20:12, Horia Geantă <horia.geanta@....com> wrote:
>>
>> On 9/14/2020 7:28 PM, Ard Biesheuvel wrote:
>>> On Mon, 14 Sep 2020 at 19:24, Horia Geantă <horia.geanta@....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.

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