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

[Jason Gunthorpe]

On Tue, Aug 25, 2026 at 03:24:01PM +0200, Andrew Jones wrote:

> The irqdomains remain per-device. The MSI table belongs to the
> corresponding S2 IOMMU domain and its configuration is bound to one VM's
> IMSIC topology. The current prototype rejects irqbypass from another VM
> once the table has an owner. The irqdomain does not sit on top of the
> vIOMMUs. A vIOMMU links guest-controlled S1 domains to their S2 parent,
> while the per-device irqdomain is the IRQ-side entry point for updating
> that parent's MSI table.

Doesn't sound like the right design to me. Pretending it is not bound
to the S2 just to make it global then blocking multi-VM is not a good
place to start from.

It needs a design where the guest remap is linked to the viommu so it
can be per-vm, somehow.

> For the host remapping implemented by this series, each irqdomain is
> per-device because it is inserted into the device's IRQ hierarchy and
> provides MSI remapping through irq_compose_msi_msg().

And this is an entirely different thing, you don't need a domain just
to call irq_compose_msi_msg()..

Jason