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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 8 Jul 2020 12:14:04 +0300
Message-ID: <CAMj1kXFGPkpaKy9NunG0sefv3bc+ETDu6H2T8RcQaKAk+tTMJg@mail.gmail.com>
Subject: Re: question regarding crypto driver DMA issue
To:     "Van Leeuwen, Pascal" <pvanleeuwen@rambus.com>
Cc:     "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>
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On Wed, 8 Jul 2020 at 11:56, Van Leeuwen, Pascal <pvanleeuwen@rambus.com> wrote:
>
> Hi,
>
> I have a question on behalf of a customer of ours trying to use the inside-secure crypto
> API driver. They are experiencing issues with result data not arriving in the result buffer.
> This seems to have something to do with not being able to DMA to said buffer, as they
> can workaround the issue by explicitly allocating a DMA buffer on the fly and copying
> data from there to the original destination.
>
> The problem I have is that I do not have access to their hardware and the driver seems
> to work just fine on any hardware  (both x64 and ARM64) I have available here, so I
> have to approach this purely theoretically ...
>
> For the situation where this problem is occuring, the actual buffers are stored inside
> the ahash_req structure. So my question is: is there any reason why this structure may
> not be DMA-able on some systems? (as I have a hunch that may be the problem ...)
>

If DMA is non-coherent, and the ahash_req structure is also modified
by the CPU while it is mapped for DMA, you are likely to get a
conflict.

It should help if you align the DMA-able fields sufficiently, and make
sure you never touch them while they are mapped for writing by the
device.