Messages in this thread | | | From | "Jason A. Donenfeld" <> | Date | Mon, 7 Nov 2016 19:23:05 +0100 | Subject | Re: [PATCH] poly1305: generic C can be faster on chips with slow unaligned access |
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On Mon, Nov 7, 2016 at 7:08 PM, Jason A. Donenfeld <Jason@zx2c4.com> 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?
In fact, I'm not so sure this happens here. In the while loop, each new walk.src.virt.addr will be aligned to BLOCK_SIZE or be aligned by virtue of being at the start of a new page. In the subsequent if block, walk.src.virt.addr will either be some_aligned_address+BLOCK_SIZE, which will be aligned, or it will be a start of a new page, which will be aligned.
So what did you have in mind exactly?
I don't think anybody is running code like:
for (size_t i = 0; i < len; i += 17) poly1305_update(&poly, &buffer[i], 17);
(And if so, those consumers should be fixed.)
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