Received-SPF: pass (google.com: domain of linux-crypto-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 2 Jan 2023 15:03:40 -0800
From:   Eric Biggers <ebiggers@kernel.org>
To:     Lukasz Stelmach <l.stelmach@samsung.com>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        linux-crypto@vger.kernel.org
Subject: Re: xor_blocks() assumptions
Message-ID: <Y7NizHFsWfMW/cC2@sol.localdomain>
References: <CGME20230102224447eucas1p1dad1a2362030eee0d3890dd3546a1532@eucas1p1.samsung.com>
 <dleftjk024o8to.fsf%l.stelmach@samsung.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <dleftjk024o8to.fsf%l.stelmach@samsung.com>
Precedence: bulk

On Mon, Jan 02, 2023 at 11:44:35PM +0100, Lukasz Stelmach wrote:
> Hi,
> 
> I am researching possibility to use xor_blocks() in crypto_xor() and
> crypto_xor_cpy(). What I've found already is that different architecture
> dependent xor functions work on different blocks between 16 and 512
> (Intel AVX) bytes long. There is a hint in the comment for
> async_xor_offs() that src_cnt (as passed to do_sync_xor_offs()) counts
> pages. Thus, it is assumed, that the smallest chunk xor_blocks() gets is
> a single page. Am I right?
> 
> Do you think adding block_len field to struct xor_block_template (and
> maybe some information about required alignment) and using it to call
> do_2 from crypto_xor() may work? I am thinking especially about disk
> encryption where sectors of 512~4096 are handled.
> 

Taking a step back, it sounds like you think the word-at-a-time XOR in
crypto_xor() is not performant enough, so you want to use a SIMD (e.g. NEON,
SSE, or AVX) implementation instead.  Have you tested that this would actually
give a benefit on the input sizes in question, especially considering that SIMD
can only be used in the kernel if kernel_fpu_begin() is executed first?

It also would be worth considering just optimizing crypto_xor() by unrolling the
word-at-a-time loop to 4x or so.

- Eric