Re: [PATCH v2 1/3] iommu/virtio: Add topology description to virtio-iommu config space

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Hi Joerg,

On 3/2/20 5:16 PM, Joerg Roedel wrote:
> On Fri, Feb 28, 2020 at 06:25:36PM +0100, Jean-Philippe Brucker wrote:
>> This solution isn't elegant nor foolproof, but is the best we can do at
>> the moment and works with existing virtio-iommu implementations. It also
>> enables an IOMMU for lightweight hypervisors that do not rely on
>> firmware methods for booting.
> 
> I appreciate the enablement on x86, but putting the conmfiguration into
> mmio-space isn't really something I want to see upstream. What is the
> problem with defining an ACPI table instead? This would also make things
> work on AARCH64 UEFI machines.
Michael has pushed this solution (putting the "configuration in the PCI
config space"), I think for those main reasons:
- ACPI may not be supported on some archs/hyps
- the virtio-iommu is a PCIe device so binding should not need ACPI
description

Another issue with ACPI integration is we have different flavours of
tables that exist: IORT, DMAR, IVRS

x86 ACPI integration was suggested with IORT. But it seems ARM is
reluctant to extend IORT to support para-virtualized IOMMU. So the VIOT
was proposed as a different atternative in "[RFC 00/13] virtio-iommu on
non-devicetree platforms"
(https://patchwork.kernel.org/cover/11257727/). Proposing a table that
may be used by different vendors seems to be a challenging issue here.

So even if the above solution does not look perfect, it looked a
sensible compromise given the above arguments. Please could you explain
what are the most compelling arguments against it?

Thanks

Eric
> 
> Regards,
> 
> 	Joerg
> 




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