Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 13 Apr 2022 at 10:47, Catalin Marinas <catalin.marinas@arm.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.

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