linux-kernel - Re: [PATCH] crypto: doc - clarify hash callbacks state machine

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Message-ID: <AM3PR04MB4520497CC3D9B2B98B8E63798D40@AM3PR04MB452.eurprd04.prod.outlook.com>
Date:   Mon, 19 Mar 2018 11:04:24 +0000
From:   Horia Geantă <horia.geanta@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
CC:     "David S. Miller" <davem@...emloft.net>,
        Jonathan Corbet <corbet@....net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] crypto: doc - clarify hash callbacks state machine

On 3/19/2018 11:25 AM, Herbert Xu wrote:
> On Mon, Mar 19, 2018 at 06:39:50AM +0000, Horia Geantă wrote:
>>
>> The fact that there can be multiple requests in parallel (for a given tfm) is a
>> different topic.
>> Each request object has its state in its own state machine, independent from the
>> other request objects.
>> I assume this is clear enough.
> 
> My point is that all of the state associated with a request needs
> to be stored in the request object.  If you're start storing things
> in the driver/hardware, then things will get ugly one way or another.
> 
Agree, the request state should be stored in the request object; I am not
debating that.

Still there are limitations even when keeping state in the request object.
For e.g. an implementation cannot DMA map a buffer for the entire lifetime of a
request object, because this lifetime is unknown - user can "abandon" the object
after a few .update() calls, or even after .init(). By "abandon" I mean not call
_ever_ any of .final(), .finup() or .export() on the object.

The only solution to avoid leaks in this case is to repeatedly DMA map & unmap
the buffer.
IOW, if one wants to load/save HW state in a buffer after an .update() and to
instruct the crypto engine to do this operation, the following steps are involved:
-gpp: DMA map the buffer, get its IOVA
-gpp: program the crypto engine with IOVA, wait for crypto engine's signal
-crypto engine: load HW state from buffer, perform the partial hash, save HW
state in buffer, signal gpp
-gpp: DMA unmap the buffer

I'd say this is pretty inefficient, yet I don't see an alternative.

Good or bad, the documentation should reflect this limitation - hence this patch.

Thanks,
Horia