Re: [PATCH 1/4] block: use ARCH_KMALLOC_MINALIGN as the default dma pad mask

[FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>]

On Wed, 21 May 2008 18:05:29 +0800
Herbert Xu <herbert@gondor.apana.org.au> wrote:

> On Wed, May 21, 2008 at 06:34:45PM +0900, FUJITA Tomonori wrote:
> >
> > Why do crypto need to know exactly the minimum alignment guaranteed by
> > kmalloc? Can you tell me an example how the alignment breaks crypto?
> 
> It's used to make the context aligned so that for most algorithms
> we can get to the context without going through ALIGN_PTR.  Only

How many bytes does the context need to be aligned?

I'm still not sure what you mean. You referred to aes, so let me use
aes as an example.

I think 'we can get to the context' means that accessing to
crypto_aes_ctx from struct crypto_tfm like this:

struct crypto_aes_ctx *ctx = crypto_tfm_ctx(tfm);


struct crypto_tfm and crypto_tfm_ctx are defined as:

struct crypto_tfm {

	u32 crt_flags;

	union {
		struct ablkcipher_tfm ablkcipher;
		struct aead_tfm aead;
		struct blkcipher_tfm blkcipher;
		struct cipher_tfm cipher;
		struct hash_tfm hash;
		struct compress_tfm compress;
	} crt_u;

	struct crypto_alg *__crt_alg;

	void *__crt_ctx[] CRYPTO_MINALIGN_ATTR;
};

static inline void *crypto_tfm_ctx(struct crypto_tfm *tfm)
{
	return tfm->__crt_ctx;
}

struct crypto_aes_ctx is placed right after struct crypto_tfm.

My question is why __crt_ctx needs ARCH_KMALLOC_MINALIGN alignment,
e.g., could be 128 bytes.


> algorithms requiring alignments bigger than that offered by kmalloc
> would have to use ALIGN_PTR.  This is important because the context
> is used on the fast path, i.e., for AES every block has to access
> the context.

Why do algorithms require alignments bigger than ARCH_KMALLOC_MINALIGN?


> If we used an alignment value is bigger than that guaranteed by
> kmalloc then this would break because the context may end up
> unaligned.
> 
> Cheers,
> -- 
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> Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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