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

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

On Tue, Mar 03, 2020 at 09:00:05AM -0500, Michael S. Tsirkin wrote:
> Not necessarily. E.g. some power systems have neither.
> There are also systems looking to bypass ACPI e.g. for boot speed.

If there is no firmware layer between the hardware and the OS the
necessary information the OS needs to run on the hardware is probably
hard-coded into the kernel? In that case the same can be done with
virtio-iommu tolopology.

> That sentence doesn't really answer the question, does it?

To be more elaborate, putting this information into config space is a
layering violation. Hardware is never completly self-descriptive and
that is why there is the firmware which provides the information about
the hardware to the OS in a generic way.

> Frankly with platform specific interfaces like ACPI, virtio-iommu is
> much less compelling.  Describing topology as part of the device in a
> way that is first, portable, and second, is a good fit for hypervisors,
> is to me one of the main reasons virtio-iommu makes sense at all.

Virtio-IOMMU makes sense in the first place because it is much faster
than emulating one of the hardware IOMMUs. And an ACPI table is also
portable to all ACPI platforms, same with device-tree.

Regards,

	Joerg