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: <38f9904b-5bf7-ea99-ed8a-27cb49f405bd@nxp.com>
Date:   Mon, 14 Sep 2020 20:12:14 +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 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.

> 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