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

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Date:   Tue, 12 Apr 2022 17:40:58 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, 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-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        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 Tue, Apr 12, 2022 at 10:32:58AM +0100, Catalin Marinas wrote:
>
> At a quick grep, most cra_alignmask values are currently 15 or smaller.
> I'm not convinced the driver needs to know about the CPU cache
> alignment. We could set cra_alignmask to CRYPTO_MINALIGN but that would
> incur unnecessary overhead via function like setkey_unaligned() when the
> arch_kmalloc_minalign() was already sufficient for DMA safety.
> 
> Maybe I miss some use-cases or I focus too much on DMA safety.

The alignment was never designed for DMA.  It's mainly for CPUs
that provide accelerated instructions that require a certain
amount of alignment, most commonly 16 bytes.

Therefore CRYPTO_MINALIGN was never meant to be a guarantee for
DMA alignment.  Any drivers relying on this is simply broken.

I understand that on ARM for historical reasons you have had a
situation that CRYPTO_MINALIGN happened to be large enough to
guarantee DMA alignment.  I totally agree with your patch to
try to fix this but it should not penalise other architectures
and raise the CRYPTO_MINALIGN unnecessarily.

I think if CRYPTO_MINALIGN makes those drivers work then so
should cra_alignmask.  And that would be a relatively easy
patch to do.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt