From: "Nicolas, Mario" <mario.nicolas@intel.com>
Subject: cra_priority usage
Date: Wed, 31 Mar 2010 17:29:13 +0100
Message-ID: <44813D8942D35947805CBEEA94195BB0013E4145A2@irsmsx505.ger.corp.intel.com>
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To: "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>
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Hi,
 
I have written a module that implements GCM algorithm for LKCF (by calling dedicated hardware).

I would like to understand how the priority mechanism works in order to be sure that my module will be selected when I need to. At the moment it seems it is always used, which is great but I'd like to understand why :)
 

Here is the crypto_alg structure I am using to register the algorithm: 

static struct crypto_alg aead_authenc_aes_gcm = {
        .cra_name               =       "rfc4106(gcm(aes))",
        .cra_driver_name        =       "icp_aead",
        .cra_priority           =       ICP_AES_ASYNC_PRIORITY, /* 3001*/
        .cra_flags              =       CRYPTO_ALG_TYPE_AEAD|CRYPTO_ALG_GENIV|CRYPTO_ALG_ASYNC,
        .cra_blocksize          =       AES_BLOCK_SIZE /*16*/,
        .cra_ctxsize            =       0,
        .cra_type               =       &crypto_aead_type,
        .cra_module             =       THIS_MODULE,
        .cra_list               =       LIST_HEAD_INIT(aead_authenc_aes_gcm.cra_list),
        .cra_exit               =       aead_auth_exit,
        .cra_u                  =       {
        .aead = {
                                    .ivsize       = AES_GCM_IV_SIZE, /*8*/
                                    .maxauthsize  = AES_GCM_AUTH_TAG_LEN, /*16*/
                                    .setkey       = setkey_aes_gcm,
                                    .setauthsize  = qat_setauthsize,
                                    .encrypt      = encrypt_aes_gcm,
                                    .decrypt      = decrypt_aes_gcm,
                                    .givencrypt   = geniv_encrypt_aes_gcm,
                                    .givdecrypt   = geniv_decrypt_aes_gcm,
                }
        }
};
 

The algorithm is registered by calling crypto_register_alg in the module_init function.

It seems that my module is always used regardless of the value of the priority (I tried to set it to 0 or even to -1).
I have also tried to load the default gcm kernel module before (or after) mine but my module is selected in both cases.

I have checked in the kernel code and I did not see any location where cra_priority is used.

So my questions are: 
-What is this variable used for?
-If there are multiple implementations of the same algorithm, how is one version chosen as opposed to another one?

I guess that the reason is that an asynchronous version always has a higher priority than a synchronous one. Is that correct?

Thanks

Mario
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