On Sat, Sep 8, 2018 at 3:35 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> Hi Ondrej,
>
> On Wed, Sep 05, 2018 at 01:30:39PM +0200, Ondrej Mosnacek wrote:
> > Since commit acb9b159c784 ("crypto: gf128mul - define gf128mul_x_* in
> > gf128mul.h"), the gf128mul_x_*() functions are very fast and therefore
> > caching the computed XTS tweaks has only negligible advantage over
> > computing them twice.
> >
> > In fact, since the current caching implementation limits the size of
> > the calls to the child ecb(...) algorithm to PAGE_SIZE (usually 4096 B),
> > it is often actually slower than the simple recomputing implementation.
> >
> > This patch simplifies the XTS template to recompute the XTS tweaks from
> > scratch in the second pass and thus also removes the need to allocate a
> > dynamic buffer using kmalloc().
> >
> > As discussed at [1], the use of kmalloc causes deadlocks with dm-crypt.
> >
> > PERFORMANCE RESULTS
> > I measured time to encrypt/decrypt a memory buffer of varying sizes with
> > xts(ecb-aes-aesni) using a tool I wrote ([2]) and the results suggest
> > that after this patch the performance is either better or comparable for
> > both small and large buffers. Note that there is a lot of noise in the
> > measurements, but the overall difference is easy to see.
> >
> > Old code:
> > ALGORITHM KEY (b) DATA (B) TIME ENC (ns) TIME DEC (ns)
> > xts(aes) 256 64 331 328
> > xts(aes) 384 64 332 333
> > xts(aes) 512 64 338 348
> > xts(aes) 256 512 889 920
> > xts(aes) 384 512 1019 993
> > xts(aes) 512 512 1032 990
> > xts(aes) 256 4096 2152 2292
> > xts(aes) 384 4096 2453 2597
> > xts(aes) 512 4096 3041 2641
> > xts(aes) 256 16384 9443 8027
> > xts(aes) 384 16384 8536 8925
> > xts(aes) 512 16384 9232 9417
> > xts(aes) 256 32768 16383 14897
> > xts(aes) 384 32768 17527 16102
> > xts(aes) 512 32768 18483 17322
> >
> > New code:
> > ALGORITHM KEY (b) DATA (B) TIME ENC (ns) TIME DEC (ns)
> > xts(aes) 256 64 328 324
> > xts(aes) 384 64 324 319
> > xts(aes) 512 64 320 322
> > xts(aes) 256 512 476 473
> > xts(aes) 384 512 509 492
> > xts(aes) 512 512 531 514
> > xts(aes) 256 4096 2132 1829
> > xts(aes) 384 4096 2357 2055
> > xts(aes) 512 4096 2178 2027
> > xts(aes) 256 16384 6920 6983
> > xts(aes) 384 16384 8597 7505
> > xts(aes) 512 16384 7841 8164
> > xts(aes) 256 32768 13468 12307
> > xts(aes) 384 32768 14808 13402
> > xts(aes) 512 32768 15753 14636
> >
> > [1] https://lkml.org/lkml/2018/8/23/1315
> > [2] https://gitlab.com/omos/linux-crypto-bench
> >
> > Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> > ---
> > crypto/xts.c | 265 ++++++++-------------------------------------------
> > 1 file changed, 39 insertions(+), 226 deletions(-)
> >
> > Changes in v3:
> > - add comment explaining the new approach as suggested by Eric
> > - ensure correct alignment in second xor_tweak() pass
> > - align performance results table header to the right
> >
> > v2: https://www.spinics.net/lists/linux-crypto/msg34799.html
> > Changes in v2:
> > - rebase to latest cryptodev tree
> >
> > v1: https://www.spinics.net/lists/linux-crypto/msg34776.html
> >
> > diff --git a/crypto/xts.c b/crypto/xts.c
> > index ccf55fbb8bc2..24cfecdec565 100644
> > --- a/crypto/xts.c
> > +++ b/crypto/xts.c
> > @@ -26,8 +26,6 @@
> > #include <crypto/b128ops.h>
> > #include <crypto/gf128mul.h>
> >
> > -#define XTS_BUFFER_SIZE 128u
> > -
> > struct priv {
> > struct crypto_skcipher *child;
> > struct crypto_cipher *tweak;
> > @@ -39,19 +37,7 @@ struct xts_instance_ctx {
> > };
> >
> > struct rctx {
> > - le128 buf[XTS_BUFFER_SIZE / sizeof(le128)];
> > -
> > le128 t;
> > -
> > - le128 *ext;
> > -
> > - struct scatterlist srcbuf[2];
> > - struct scatterlist dstbuf[2];
> > - struct scatterlist *src;
> > - struct scatterlist *dst;
> > -
> > - unsigned int left;
> > -
> > struct skcipher_request subreq;
> > };
> >
> > @@ -96,265 +82,92 @@ static int setkey(struct crypto_skcipher *parent, const u8 *key,
> > return err;
> > }
> >
> > -static int post_crypt(struct skcipher_request *req)
> > +/*
> > + * We compute the tweak masks twice (both before and after the ECB encryption or
> > + * decryption) to avoid having to allocate a temporary buffer and/or make
> > + * mutliple calls to the 'ecb(..)' instance, which usually would be slower than
> > + * just doing the gf128mul_x_ble() calls again.
> > + */
> > +static int xor_tweak(struct skcipher_request *req, struct skcipher_request *subreq)
> > {
> > struct rctx *rctx = skcipher_request_ctx(req);
> > - le128 *buf = rctx->ext ?: rctx->buf;
> > - struct skcipher_request *subreq;
> > + struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
> > const int bs = XTS_BLOCK_SIZE;
> > struct skcipher_walk w;
> > - struct scatterlist *sg;
> > - unsigned offset;
> > + le128 t = rctx->t;
> > int err;
> >
> > - subreq = &rctx->subreq;
> > + /* set to our TFM to enforce correct alignment: */
> > + skcipher_request_set_tfm(subreq, tfm);
> > +
> > err = skcipher_walk_virt(&w, subreq, false);
> >
>
> Hmm, it confused me how 'subreq' isn't necessarily the same as 'rctx->subreq'.
> Also skcipher_request_set_tfm() is called even on the original 'req'. I suppose
> it ends up setting it to the previous value and therefore is safe, but I'm not
> completely sure; do any other algorithms do that? I don't think it's a good
> idea in general to modify the request besides the request_ctx() portion.
I don't think it can do any harm, skcipher_request_set_tfm() is just:
static inline void skcipher_request_set_tfm(struct skcipher_request *req,
struct crypto_skcipher *tfm)
{
req->base.tfm = crypto_skcipher_tfm(tfm);
}
However I agree it is a bit hacky...
>
> Actually all the information is available from the original 'req' anyway, so why
> not just pass a bool that indicates whether it's the first or second XOR pass?
> Like the following incremental patch:
I generally try to avoid bool arguments (or at least too many of them)
if possible, since it is easy to get lost in what the 0s and 1s mean
in a function call, but I concur that in this case a bool argument is
better than the alternative. I like your patch, I am going to fold it
in, thanks!
>
> diff --git a/crypto/xts.c b/crypto/xts.c
> index 24cfecdec5656..0df868aa0ae7f 100644
> --- a/crypto/xts.c
> +++ b/crypto/xts.c
> @@ -88,7 +88,7 @@ static int setkey(struct crypto_skcipher *parent, const u8 *key,
> * mutliple calls to the 'ecb(..)' instance, which usually would be slower than
> * just doing the gf128mul_x_ble() calls again.
> */
> -static int xor_tweak(struct skcipher_request *req, struct skcipher_request *subreq)
> +static int xor_tweak(struct skcipher_request *req, bool second_pass)
> {
> struct rctx *rctx = skcipher_request_ctx(req);
> struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
> @@ -97,10 +97,12 @@ static int xor_tweak(struct skcipher_request *req, struct skcipher_request *subr
> le128 t = rctx->t;
> int err;
>
> - /* set to our TFM to enforce correct alignment: */
> - skcipher_request_set_tfm(subreq, tfm);
> -
> - err = skcipher_walk_virt(&w, subreq, false);
> + if (second_pass) {
> + req = &rctx->subreq;
> + /* set to our TFM to enforce correct alignment: */
> + skcipher_request_set_tfm(req, tfm);
> + }
> + err = skcipher_walk_virt(&w, req, false);
>
> while (w.nbytes) {
> unsigned int avail = w.nbytes;
> @@ -124,11 +126,9 @@ static int xor_tweak(struct skcipher_request *req, struct skcipher_request *subr
> static void crypt_done(struct crypto_async_request *areq, int err)
> {
> struct skcipher_request *req = areq->data;
> - struct rctx *rctx = skcipher_request_ctx(req);
> - struct skcipher_request *subreq = &rctx->subreq;
>
> if (!err)
> - err = xor_tweak(req, subreq);
> + err = xor_tweak(req, true);
>
> skcipher_request_complete(req, err);
> }
> @@ -154,9 +154,9 @@ static int encrypt(struct skcipher_request *req)
> struct skcipher_request *subreq = &rctx->subreq;
>
> init_crypt(req);
> - return xor_tweak(req, req) ?:
> + return xor_tweak(req, false) ?:
> crypto_skcipher_encrypt(subreq) ?:
> - xor_tweak(req, subreq);
> + xor_tweak(req, true);
> }
>
> static int decrypt(struct skcipher_request *req)
> @@ -165,9 +165,9 @@ static int decrypt(struct skcipher_request *req)
> struct skcipher_request *subreq = &rctx->subreq;
>
> init_crypt(req);
> - return xor_tweak(req, req) ?:
> + return xor_tweak(req, false) ?:
> crypto_skcipher_decrypt(subreq) ?:
> - xor_tweak(req, subreq);
> + xor_tweak(req, true);
> }
>
> static int init_tfm(struct crypto_skcipher *tfm)
--
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.
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