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: <CAMj1kXGg7bSh57kwE57mKRocNRPZCeXifwjF53-3Jb6LYsfZTg@mail.gmail.com>
Date:   Mon, 14 Sep 2020 19:28:29 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Horia Geantă <horia.geanta@....com>
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 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.

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.