linux-kernel - Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC

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Date:   Thu, 14 Apr 2022 16:30:54 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

On Wed, 13 Apr 2022 at 10:47, Catalin Marinas <catalin.marinas@....com> wrote:
..
>
> Let's go back to restating the crypto code alignment requirements, as I
> understand them (please correct):
>
> 1. Bus master accesses (DMA, CPUs that can't do efficient unaligned
>    accesses). That's what cra_alignmask is for. If there's a driver that
>    relies on an implicit kmalloc() alignment higher than cra_alignmask,
>    it is already broken on x86 and a few other architectures that don't
>    define ARCH_DMA_MINALIGN. But it won't be any worse with this series
>    since it only brings the arm64 kmalloc() alignment down from 128 to
>    64.
>
> 2. Non-coherent DMA and cache invalidation. With my series, kmalloc'ed
>    buffers are DMA-safe for the CPU/SoC the kernel is running on.
>
> 3. DMA into buffers embedded in TFM structures. Since the offset of
>    those buffers is decided at compile-time, we need a
>    CRYPTO_MINALIGN_ATTR that covers both bus master alignment
>    requirements and the highest cache line size (CWG) for a non-coherent
>    DMA SoC. Ard's series would simplify the latter but, before then,
>    CRYPTO_MINALIGN needs to stay as the larger ARCH_DMA_MINALIGN.
>
> With my series, there is no change to the value of CRYPTO_MINALIGN for
> arm64 or any other architecture, so point 3 is unaffected. The series
> does change the kmalloc() alignment and that may be smaller than
> CRYPTO_MINALIGN but neither of points 1 or 2 above are affected since
> (a) we still have a sufficiently large ARCH_KMALLOC_MINALIGN of 64 and
> (b) the kmalloc'ed buffers are safe for non-coherent DMA.
>
> Herbert, Ard, if I missed anything please let me know but based on my
> understanding, this series is safe for the crypto code.
>

Agreed.