Re: [PATCH] poly1305: generic C can be faster on chips with slow unaligned access

[Eric Biggers <ebiggers@google.com>]

On Mon, Nov 07, 2016 at 07:08:22PM +0100, Jason A. Donenfeld wrote:
> Hmm... The general data flow that strikes me as most pertinent is
> something like:
> 
> struct sk_buff *skb = get_it_from_somewhere();
> skb = skb_share_check(skb, GFP_ATOMIC);
> num_frags = skb_cow_data(skb, ..., ...);
> struct scatterlist sg[num_frags];
> sg_init_table(sg, num_frags);
> skb_to_sgvec(skb, sg, ..., ...);
> blkcipher_walk_init(&walk, sg, sg, len);
> blkcipher_walk_virt_block(&desc, &walk, BLOCK_SIZE);
> while (walk.nbytes >= BLOCK_SIZE) {
>     size_t chunk_len = rounddown(walk.nbytes, BLOCK_SIZE);
>     poly1305_update(&poly1305_state, walk.src.virt.addr, chunk_len);
>     blkcipher_walk_done(&desc, &walk, walk.nbytes % BLOCK_SIZE);
> }
> if (walk.nbytes) {
>     poly1305_update(&poly1305_state, walk.src.virt.addr, walk.nbytes);
>     blkcipher_walk_done(&desc, &walk, 0);
> }
> 
> Is your suggestion that that in the final if block, walk.src.virt.addr
> might be unaligned? Like in the case of the last fragment being 67
> bytes long?

I was not referring to any users in particular, only what users could do.  As an
example, if you did crypto_shash_update() with 32, 15, then 17 bytes, and the
underlying algorithm is poly1305-generic, the last block would end up
misaligned.  This doesn't appear possible with your pseudocode because it only
passes in multiples of the block size until the very end.  However I don't see
it claimed anywhere that shash API users have to do that.

Eric