Re: [PATCH v1 1/1] dm-integrity: Implement asynch digest support

Harald Freudenberger
2025-01-16T07:33:59Z

On 2025-01-15 18:37, Eric Biggers wrote:
> On Wed, Jan 15, 2025 at 05:46:57PM +0100, Harald Freudenberger wrote:
>> Use the async digest in-kernel crypto API instead of the
>> synchronous digest API. This has the advantage of being able
>> to use synchronous as well as asynchronous digest implementations
>> as the in-kernel API has an automatic wrapping mechanism
>> to provide all synchronous digests via the asynch API.
>>
>> Tested with crc32, sha256, hmac-sha256 and the s390 specific
>> implementations for hmac-sha256 and protected key phmac-sha256.
>>
>> Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
>
> As Mikulas mentioned, this reduces performance for everyone else, which
> is not
> great. It also makes the code more complicated.
>
> I also see that you aren't actually using the algorithm in an async
> manner, but
> rather waiting for it synchronously each time. Thus the ability to
> operate
> asynchronously provides no benefit in this case, and this change is
> purely about
> allowing a particular driver to be used, presumably the s390 phmac one
> from your
> recent patchset. Since s390 phmac seems to be new code, and
> furthermore it is
> CPU-based and thus uses virtual addresses (which makes the use of
> scatterlists
> entirely pointless), wouldn't it be easier to just make it implement
> shash
> instead of ahash, moving any wait that may be necessary into the driver
> itself?
>
> - Eric

Thanks for this feedback. I'll give it a try with some performance
measurements.
And I totally agree that a synchronous implementation of phmac whould
have solved
this also. But maybe you can see that this is not an option according to
Herbert Xu's feedback about my first posts with implementing phmac as an
shash.
The thing is that we have to derive a hardware based key (pkey) from the
given key material and that may be a sleeping call which a shash must
not invoke.
So finally the phmac implementation is now an ahash digest
implementation
as suggested by Herbert.

You are right, my patch is not really asynchronous. Or at least waiting
for
completion at the end of each function. However, opposed to the ahash
invocation
where there have been some update() calls this is now done in just one
digest()
giving the backing algorithm a chance to hash all this in one step (well
it still
needs to walk the scatterlist).

Is there a way to have dm-integrity accept both, a ahash() or a shash()
digest?