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Subject: Re: [PATCH v6 0/7] Add virtio-iommu driver

On 19/12/2018 23:09, Michael S. Tsirkin wrote:
> On Thu, Dec 13, 2018 at 12:50:29PM +0000, Jean-Philippe Brucker wrote:
>>>> [3] git://linux-arm.org/linux-jpb.git virtio-iommu/v0.9.1
>>>> ÂÂÂÂ git://linux-arm.org/kvmtool-jpb.git virtio-iommu/v0.9
>>> Unfortunatly gitweb seems to be broken on linux-arm.org. What is missing
>>> in this patch-set to make this work on x86?
>> You should be able to access it here:
>> http://www.linux-arm.org/git?p=linux-jpb.git;a=shortlog;h=refs/heads/virtio-iommu/devel
>> That branch contains missing bits for x86 support:
>> * ACPI support. We have the code but it's waiting for an IORT spec
>> update, to reserve the IORT node ID. I expect it to take a while, given
>> that I'm alone requesting a change for something that's not upstream or
>> in hardware.
> Frankly I think you should take a hard look at just getting the data
> needed from the PCI device itself.  You don't need to depend on virtio,
> it can be a small driver that gets you that data from the device config
> space and then just goes away.
> If you want help with writing such a small driver let me know.
> If there's an advantage to virtio-iommu then that would be its
> portability, and it all goes out of the window because
> of dependencies on ACPI and DT and OF and the rest of the zoo.

But the portable solutions are ACPI and DT.

Describing the DMA dependency through a device would require the guest
to probe the device before all others. How do we communicate this?
* pass a kernel parameter saying something like "probe_first=00:01.0"
* make sure that the PCI root complex is probed before any other
platform device (since the IOMMU can manage DMA of platform devices).
* change DT, ACPI and PCI core code to handle this probe_first kernel

Better go with something standard, that any OS and hypervisor knows how
to use, and that other IOMMU devices already use.

>> * DMA ops for x86 (see "HACK" commit). I'd like to use dma-iommu but I'm
>> not sure how to implement the glue that sets dma_ops properly.
>> Thanks,
>> Jean
> OK so IIUC you are looking into Christoph's suggestions to fix that up?

Eventually yes. I'll give it a try next year, once the dma-iommu changes
are on the list. It's not a priority for me, given that x86 already has
a pvIOMMU with VT-d, and that Arm still needs one. It shouldn't block
this series.

> There's still a bit of time left before the merge window,
> maybe you can make above changes.

I'll wait to see if Joerg has other concerns about the design or the
code, and resend in January. I think that IOMMU driver changes should go
through his tree.


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