Re: [PATCH v3 1/3] crypto: qat - use pre-allocated buffers in datapath

[Lucas Segarra <lucas.segarra.fernandez@intel.com>]

On Mon, Jul 11, 2022 at 03:21:39PM +0100, Giovanni Cabiddu wrote:
> On Mon, Apr 11, 2022 at 05:37:24PM +0000, Eric Biggers wrote:
> > On Sun, Apr 10, 2022 at 08:47:05PM +0100, Giovanni Cabiddu wrote:
> > > If requests exceed 4 entries buffers, memory is allocated dynamically.
> > > 
> > > In addition, remove the CRYPTO_ALG_ALLOCATES_MEMORY flag from both aead
> > > and skcipher alg structures.
> > > 
> > 
> > There is nothing that says that algorithms can ignore
> > !CRYPTO_ALG_ALLOCATES_MEMORY if there are too many scatterlist entries.  See the
> > comment above the definition of CRYPTO_ALG_ALLOCATES_MEMORY.
> > 
> > If you need to introduce this constraint, then you will need to audit the users
> > of !CRYPTO_ALG_ALLOCATES_MEMORY to verify that none of them are issuing requests
> > that violate this constraint, then add this to the documentation comment for
> > CRYPTO_ALG_ALLOCATES_MEMORY.
> Belatedly...
> 
> Adding to this thread my colleague Lucas who did an audit of the users
> of !CRYPTO_ALG_ALLOCATES_MEMORY to understand if we can add a constraint
> to the definition of CRYPTO_ALG_ALLOCATES_MEMORY.
> 
> Regards,
> 
> -- 
> Giovanni

An audit was done on users of !CRYPTO_ALG_ALLOCATES_MEMORY: dm-crypt and dm-integrity. dm-crypt uses scatterlists with at most 4 entries, but dm-integrity may allocate memory for scatterlist with arch-dependent and system-bounded number of entries. Therefore the constraint in https://lore.kernel.org/linux-crypto/20200722072932.GA27544@gondor.apana.org.au/ cannot be introduced.

A way to solve the problem might be to forward requests with more than 4 entries in a scatterlist to an implementation that does not allocate memory. This will introduce always a performance penalty for requests with scatterlists greater than 4 in algorithms backed up by HW accelerators, even if the requestor does not requires this restriction. A way to solve this might be to register two versions of the same algorithm, one without CRYPTO_ALG_ALLOCATES_MEMORY that forwards to SW and one with CRYPTO_ALG_ALLOCATES_MEMORY set that doesn’t. Any suggestions?

Adding Horia Geantă and dm-devel based on the previous thread.

Thanks.
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