linux-kernel - Re: Can crypto API provide information about hw acceleration?

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Message-ID: <20190514213409.GA115510@gmail.com>
Date:   Tue, 14 May 2019 14:34:10 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     dsterba@...e.cz, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Can crypto API provide information about hw acceleration?

On Tue, May 14, 2019 at 06:33:48PM +0200, David Sterba wrote:
> Hi,
> 
> Q: is there a way to query the crypto layer whether a given algorithm
> (digest, crypto) is accelerated by the driver?
> 
> This information can be used to decide if eg. a checksum should can be
> calculated right away or offloaded to a thread. This is done in btrfs,
> (fs/btrfs/disk-io.c:check_async_write).
> 
> At this moment it contains a static check for a cpu feature, and only
> for x86. I briefly searched the arch/ directory for implementations of
> crc32c that possibly use hw aid and there are several of them. Adding a
> static check a-la x86 for the other architectures (arm, ppc, mips,
> sparc, s390) is wrong, so I'm looking for a clean solution.
> 
> The struct shash_alg definition of the algorithms does not say anything
> about the acceleration. The closest thing is the cra_priority, but I
> don't know if this is reliable information. The default implementations
> seem to have 100, and acceleated 200 or 300.
> 
> This would be probably sufficient, but I'd like a confirmation from
> crypto people.
> 

There's only one default implementation of crc32c, not multiple, and it has
priority 100.  All other crc32c implementations have priority > 100.  So yes,
you can check the priority (which would require adding a function to
lib/libcrc32c.c to get it).  Alternatively you could check whether the driver
name is "crc32c-generic" or not.

- Eric