dm crypt: log the encryption algorithm implementation

Log the encryption algorithm's driver name when a dm-crypt target is
created.  This will help people determine whether the expected
implementation is being used.  In some cases we've seen people do
benchmarks and reject using encryption for performance reasons, when in
fact they used a much slower implementation than was possible on the
hardware.  It can make an enormous difference; e.g., AES-XTS on ARM can
be over 10x faster with the crypto extensions than without.  It can also
be useful to know if an implementation using an external crypto
accelerator is being used instead of a software implementation.

Example message:

[   29.307629] device-mapper: crypt: xts(aes) using implementation "xts-aes-ce"

We've already found the similar message in fs/crypto/keyinfo.c to be
very useful.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-crypt.c

@@ -1885,6 +1885,13 @@ static int crypt_alloc_tfms_skcipher(struct crypt_config *cc, char *ciphermode)
 		}
 	}
 
+	/*
+	 * dm-crypt performance can vary greatly depending on which crypto
+	 * algorithm implementation is used.  Help people debug performance
+	 * problems by logging the ->cra_driver_name.
+	 */
+	DMINFO("%s using implementation \"%s\"", ciphermode,
+	       crypto_skcipher_alg(any_tfm(cc))->base.cra_driver_name);
 	return 0;
 }
 
@@ -1903,6 +1910,8 @@ static int crypt_alloc_tfms_aead(struct crypt_config *cc, char *ciphermode)
 		return err;
 	}
 
+	DMINFO("%s using implementation \"%s\"", ciphermode,
+	       crypto_aead_alg(any_tfm_aead(cc))->base.cra_driver_name);
 	return 0;
 }