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

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Date:   Fri, 15 Apr 2022 09:49:12 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        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 Fri, 15 Apr 2022 at 08:51, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Wed, Apr 13, 2022 at 09:47:29AM +0100, Catalin Marinas wrote:
> >
> > 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.
>
> Sorry, but you can't change CRYPTO_MINALIGN to a value greater
> than the minimum alignment returned by kmalloc.  That simply
> doesn't work.  There is no magic in the Crypto API that makes
> this work.
>

I'm not sure I understand what would go wrong if that assumption no
longer holds, but if CRYPTO_MINALIGN needs to remain equal to
ARCH_KMALLOC_MINALIGN, let's at least decouple it from
ARCH_DMA_MINALIGN, as I do in my series. As I pointed out before,
ARCH_DMA_MINALIGN has nothing to do with DMA addressing capabilities
of individual masters, it is simply a worst case cacheline size that
needs to be taken into account to avoid corruption when doing cache
invalidation for non-cache coherent inbound DMA.

I'll rename the flag I proposed from CRYPTO_ALG_NEED_DMA_ALIGNMENT to
CRYPTO_ALG_NEED_DMA_PADDING to make this clearer, and given that only
a few drivers should be relying on DMA to write into request/TFM
context structures, hopefully we can fix those to stop doing that, and
get rid of this flag again entirely.