[dm-devel] [PATCH 0/5] crypto: add IV generation templates

[Ard Biesheuvel]

On 20 July 2018 at 20:45, Mark Brown <broonie at kernel.org> wrote:
> On Fri, Jul 20, 2018 at 10:02:21AM +0900, Ard Biesheuvel wrote:
>> On 20 July 2018 at 00:50, Mark Brown <broonie at kernel.org> wrote:
>
>> > Existing hardware can definitely do the IV generation and I believe that
>> > it can chain multiple sectors together though I'd need to confirm this,
>> > as mentioned elsewhere in the thread the ccree driver is for one of
>> > the relevant devices.  I've poked some relevant people.
>
>> As far as I can infer from the ccree driver source, IV generation and
>> en/decryption are separate operations, and given that each sector
>> requires both operations to be applied in sequence, letting the crypto
>> layer handle an entire bio does not have any benefit *at the moment*.
>
> Interesting...  they were reporting some benefits from that with their
> out of tree driver prior to upstreaming (and there are other
> implementations out there, that's the only one I definitely know about).
> I have to confess I didn't look at their in tree driver, looking briefly
> now it looks awfully like the hardware should be able to chain IV
> generation together with encryption without bothering the CPU which
> would be good enough.
>

Indeed interesting. But afaict, that would still mean that the IV
generation transform and the payload transform would be expressed as a
single crypto algorithm, e.g., 'dm(essiv-foo(aes),gcm(aes)), or the DM
layer would still need to be involved in sequencing one operation
after the other, and I don't think any of that support is in the
current series. But I'm just a drive by reviewer here, so please
correct me if I am wrong.

>> In fact, it seems to me that the ability to use protected AES keys is
>> much more appealing than any performance argument (including 'it may
>> be slower but at least it does not load the CPU'), so some background
>> on how such a change would enable this use case would be beneficial as
>> well to getting this adopted.
>
> Right, that's another benefit which was on the radar for followup work.
>
>> So my recommendation would be to focus on moving the IV generation
>> into the crypto layer, but conservatively, and not confuse people by
>> making additional changes that could theoretically improve
>> performance, but only on hardware that does not exist.
>
> It certainly seems like splitting things up will at least allow things
> to progress.

Indeed.