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

[Joerg Roedel <joro@8bytes.org>]

Hi Eric,

On Tue, Mar 03, 2020 at 11:19:20AM +0100, Auger Eric wrote:
> 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

But on those there is device-tree, right?

> - the virtio-iommu is a PCIe device so binding should not need ACPI
> description

The other x86 IOMMUs are PCI devices too and they definitly need a ACPI
table to be configured.

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

An integration with IORT might be the best, but if it is not possible
ther can be a new table-type for Virtio-iommu. That would still follow
platform best practices.

> 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.

Yeah, if I am reading that right this proposes a one-fits-all solution.
That is not needed as the other x86 IOMMUs already have their tables
defined and implemented. There is no need to change anything there.

> 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?

It is a hack and should be avoided if at all possible. And defining an
own ACPI table type seems very much possible.


Regards,

	Joerg
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