linux-kernel - Re: Device address specific mapping of arm,mmu-500

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Date:   Wed, 31 May 2017 13:44:19 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ray Jui <ray.jui@...adcom.com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        Robin Murphy <robin.murphy@....com>,
        Mark Rutland <mark.rutland@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Device address specific mapping of arm,mmu-500

On Tue, May 30, 2017 at 11:13:36PM -0700, Ray Jui wrote:
> I did a little more digging myself and I think I now understand what you
> meant by identity mapping, i.e., configuring the MMU-500 with 1:1 mapping
> between the DMA address and the IOVA address.
> 
> I think that should work. In the end, due to this MSI write parsing issue in
> our PCIe controller, the reason to use IOMMU is to allow the cache
> attributes (AxCACHE) of the MSI writes towards GICv3 ITS to be modified by
> the IOMMU to be device type, while leaving the rest of inbound reads/writes
> from/to DDR with more optimized cache attributes setting, to allow I/O
> coherency to be still enabled for the PCIe controller. In fact, the PCIe
> controller itself is fully capable of DMA to/from the full address space of
> our SoC including both DDR and any device memory.
> 
> The 1:1 mapping will still pose some translation overhead like you
> suggested; however, the overhead of allocating page tables and locking will
> be gone. This sounds like the best possible option I have currently.

It might end up being pretty invasive to work around a hardware bug, so
we'll have to see what it looks like. Ideally, we could just use the SMMU
for everything as-is and work on clawing back the lost performance (it
should be possible to get ~95% of the perf if we sort out the locking, which
we *are* working on).

> May I ask, how do I start to try to get this identity mapping to work as an
> experiment and proof of concept? Any pointer or advise is highly appreciated
> as you can see I'm not very experienced with this. I found Will recently
> added the IOMMU_DOMAIN_IDENTITY support to the arm-smmu driver. But I
> suppose that is to bypass the SMMU completely, instead of still going
> through the MMU with 1:1 translation. Is my understanding correct?

Yes, I don't think IOMMU_DOMAIN_IDENTITY is what you need because you
actally need per-page control of memory attributes.

Robin might have a better idea, but I think you'll have to hack dma-iommu.c
so that you can have a version of the DMA ops that:

  * Initialises the identity map (I guess as normal WB cacheable?)
  * Reserves and maps the MSI region appropriately
  * Just returns the physical address for the dma address for map requests
    (return error for the MSI region)
  * Does nothing for unmap requests

But my strong preference would be to fix the locking overhead from the
SMMU so that the perf hit is acceptable.

Will