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: <CAMj1kXFeZP7_TQ73yLq0dfb=3wMS0VxqbKKUCGny0xHW1xL+5g@mail.gmail.com>
Date:   Tue, 15 Sep 2020 13:26:25 +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 Tue, 15 Sep 2020 at 13:02, Horia Geantă <horia.geanta@....com> wrote:
>
> 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.
>

This is called once per XTS request, right? And you are saying the
extra cycle makes a difference?

> 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.

> 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.