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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Mon, 30 May 2022 12:09:04 +0200
Message-ID: <CAMj1kXEj6eGT8LszJTgBAec3Aq9tgsPC-cByGJ1vEXKny3Ui5Q@mail.gmail.com>
Subject: Re: [RFC PATCH 2/2] Removes the x86 dependency on the QAT drivers
To:     yoan.picchi@arm.com
Cc:     Andre Przywara <andre.przywara@arm.com>,
        "David S. Miller" <davem@davemloft.net>,
        Giovanni Cabiddu <giovanni.cabiddu@intel.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        qat-linux <qat-linux@intel.com>
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On Mon, 30 May 2022 at 11:58, Yoan Picchi <yoan.picchi@foss.arm.com> wrote:
>
>  > On Wed, 18 May 2022 at 17:55, Andre Przywara <andre.przywara@arm.com>
> wrote:
>  > >
>  > > On Tue, 17 May 2022 10:11:09 +0200
>  > > Ard Biesheuvel <ardb@kernel.org> wrote:
>  > >
>  > > Hi,
>  > >
>  > > > On Mon, 16 May 2022 at 12:16, <yoan.picchi@arm.com> wrote:
>  > > > >
>  > > > > From: Yoan Picchi <yoan.picchi@arm.com>
>  > > > >
>  > > > > This dependency looks outdated. After the previous patch, we
> have been able
>  > > > > to use this driver to encrypt some data and to create working
> VF on arm64.
>  > > > >
>  > > > > Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
>  > > >
>  > > > Are you sure the driver is safe for non-coherent DMA as well?
>  > >
>  > > That depends on your definition of "sure".
>  > > We indeed tested this only on a server with coherent PCIe.
>  > >
>  > > I skimmed through the driver, and it looks like to use the DMA API
>  > > correctly:
>  > > - I see dma_alloc_coherent() calls for DMA ring buffers.
>  > > - There are dma_map_single()/dma_unmap_single() pairs in other parts.
>  > > - Accesses to the BARs are capsuled via macros, using readl/writel.
>  > > - Access the the SRAM BAR is also only done via those macros.
>  > >
>  > > I didn't go through the driver systematically, and of course the
>  > > interesting parts are the ones you don't see easily, so I am eager
> to hear
>  > > any other opinions on this topic.
>  > >
>  > > Ard, do you have anything special in mind? Is there something to
> look out
>  > > for, specifically?
>  > >
>  >
>  > If it uses the DMA api consistently and correctly, and works as
>  > expected when running under a SMMU, things are probably fine
>  >
>  > > The few cards we have access to are in some server in the data
> centre, so
>  > > I can't easily walk in with, say a RockPro64, and test this there.
>  > >
>  >
>  > I suppose this implies that you have tested with SMMUs enabled.
>
> Sorry for the delay, I was away for a few days.
> Actually, our previous attempts were with the iommu set to passthrough,
> but I
> just tested without the passthrough and it works the same way.

Thanks for confirming.

So this looks fine to me as far as un-x86-like DMA topologies are
concerned. I do agree that big-endian should be forbidden or tested
thoroughly as well.