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

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Date:   Thu, 7 Apr 2022 12:01:02 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
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 Thu, Apr 07, 2022 at 02:30:54PM +1000, Herbert Xu wrote:
> On Wed, Apr 06, 2022 at 09:49:42AM +0100, Catalin Marinas wrote:
> > is any change to the crypto code.
> 
> But the crypto API assumes that memory returned by kmalloc is
> automatically aligned to CRYPTO_MINALIGN, would this still be
> the case if you change it to ARCH_DMA_MINALIGN?

No but I think that's a valid point. Taking the crypto_tfm structure as
an example with ARCH_DMA_MINALIGN of 128:

#define CRYPTO_MINALIGN 128
#define CRYPTO_MINALIGN_ATTR __attribute__ ((__aligned__(CRYPTO_MINALIGN)))

struct crypto_tfm {
	u32 crt_flags;
	int node;
	void (*exit)(struct crypto_tfm *tfm);
	struct crypto_alg *__crt_alg;
	void *__crt_ctx[] CRYPTO_MINALIGN_ATTR;
};

The alignof(struct crypto_tfm) is 128. However, a kmalloc() would only
guarantee the smaller ARCH_KMALLOC_MINALIGN which, after this series,
would be 64 for arm64. From the DMA perspective there's no issue with
the smaller kmalloc() alignment since, if a crypto_tfm pointer is
DMA-aligned for the hardware it is running on, so would __ctr_ctx[] at
an offset multiple of the dynamic DMA alignment. If we used
ARCH_KMALLOC_MINALIGN instead and the hardware alignment requirement was
larger, than we would have a potential problem with non-coherent DMA.

The only issue is whether the compiler gets confused by a pointer to a
structure with a smaller alignment than alignof(struct ...). I don't see
a performance or correctness issue on arm64 here. It would be a problem
if instead of 16 we went down to 8 or 4 due to unaligned accesses but
from 128 to 64 (or even 16), I don't think it matters.

-- 
Catalin