netdev - Re: [PATCH v2 net-next] tcp: md5: use kmalloc() backed scratch areas

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Date:	Wed, 29 Jun 2016 10:23:37 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] tcp: md5: use kmalloc() backed scratch areas

On Tue, Jun 28, 2016 at 10:35:31AM -0700, Andy Lutomirski wrote:
>
> Do you mean this code:

Yes.
 
> I'm wondering why support for scatterlists is all-or-nothing.  Why
> can't we initialize a hash object and then alternate between passing
> it scatterlists and pointers?

Because once you have started hashing the hash state is not stored
in a consistent format.  Our software code may maintain one format
while a hardware implementation could do something else altogether.
So you have to stick with one implementation throughout a particular
hashing session.

> I'm guessing that ahash enables async operation and shash is
> synchronous only.  If I'm right, I understand why ahash requires a
> scatterlist.  What I don't understand is why shash can't also accept a
> scatterlist.  It appears that most of the ahash users in the tree
> actually want synchronous crypto and are presumably using ahash for
> some other reason such as ahash's ability to hash via scatterlist (in
> this case, struct page *).

ahash is meant to be the interface everyone uses regardless of
whether they want sync-only or async.  shash should only be used
for small amounts of hashing on virtual addresses.

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