Re: [RFC PATCH 5/5] hw/arm/virt-acpi-build: Add IORT RMR regions to handle MSI nested binding

[Nicolin Chen]

Hi Eric/Don/Shameer,

On Fri, Nov 08, 2024 at 12:52:42PM +0000, Shameer Kolothum wrote:
> Those IOVAs belong to [0x8000000, 0x8100000] matching
> MSI_IOVA_BASE and MSI_IOVA_LENGTH definitions in kernel
> arm-smmu-v3 driver. This is the window used to allocate
> IOVAs matching physical MSI doorbells.
[...]
> +static void
> +build_iort_rmr_nodes(GArray *table_data, GArray *smmu_idmaps,
> +                     size_t *smmu_offset, uint32_t *id)
> +{
[...]
> +        /* Physical Range offset */
> +        build_append_int_noprefix(table_data, 0x8000000, 8);
> +        /* Physical Range length */
> +        build_append_int_noprefix(table_data, 0x100000, 8);
> +        build_append_int_noprefix(table_data, 0, 4); /* Reserved */
> +        *id += 1;
> +    }

Jason made some kernel patches for iommufd to do MSI mappings:
https://github.com/jgunthorpe/linux/commits/for-nicolin/
It addresses Robin's remark against a get_msi_mapping_domain API
so that we could likely support a RMR solution as well, if a VMM
chooses to use it v.s. a future non-RMR one (mapping vITS page).

Yet, here we seem to be missing a pathway between VMM and kernel
to agree on the MSI window decided by the kernel, as this patch
does the hard coding for a [0x8000000, 0x8100000) range.

Though I am aware that the sysfs node
"/sys/devices/pci000x/000x/iommu_group/reserved_regions" exposes
the MSI window, it's probably better to have a new iommufd uAPI
to expose the range, so a nested domain eventually might be able
to choose between a RMR flow and a non-RMR flow.

I have been going through the structures between QEMU's SMMU code
and virt/virt-acpi-build code, yet having a hard time to figure
out a way to forward the MSI window from the SMMU code to IORT,
especially after this series changes the "smmu" instance creation
from virt code to "-device" string. Any thought?

Thanks
Nicolin