Re: [PATCH v4 02/21] iommufd: Add iommufd_sw_map_msi()

[Jason Gunthorpe]

On Wed, Aug 26, 2026 at 10:23:20AM +0200, Andrew Jones wrote:
> RISC-V uses the same basic model as x86 (an IOMMU-owned interrupt
> remapping IRQ domain). x86 makes that domain per-IOMMU because its IRTE
> resources belong to the IOMMU instance. RISC-V makes it per-device because
> its callbacks operate on the device's attached IOMMU domain and preserve
> that device's existing MSI parent. The IRQ callbacks provide the lifecycle
> hooks, even though the mappings, MSI table, and device context all remain
> IOMMU state.

The irq domain isn't per device, it is per msi table, many devices can
share the same table, and the VM should have a single shared table
across all devices. That's the core uniquely RISCV wrinkle I think.

IDK how Thomas would like to model this, but you will get a better
result if it is very clearly explained and asked.

I would like to keep the iommu out of it, so I prefer if IRQ code
patches in its MSI table pointer broadly the same way AMD does.
Though AMD doesn't have the restriction that remapping only works with
a S2 domain :(

Jason