Re: [PATCH rfcv3 15/21] intel_iommu: Bind/unbind guest page table to host

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Jun 12, 2025 at 02:06:15PM +0000, Shameerali Kolothum Thodi wrote:
> > >> The "switch" in vSMMU model is only needed by KVM for MSI doorbell
> > >> translation. By thinking it carefully, maybe it shouldn't switch AS
> > >> because VFIO might be confused if it somehow does get_address_space
> > >> again in the future..
> > 
> > @Nicolin, not quite get the detailed logic for the MSI stuff on SMMU. But I
> > agree with the last sentence. get_address_space should return a consistent
> > AS.
> 
> I think it is because, in ARM world the MSI doorbell address is translated by
> an IOMMU. Hence, if the Guest device is behind IOMMU, it needs to return
> the IOMMU AS in,
> 
> kvm_irqchip_add_msi_route()
>  kvm_arch_fixup_msi_route()
>    pci_device_iommu_address_space()  --> .get_address_space()  -->At this point we now return IOMMU AS.
> 
> If not the device will be configured with a  wrong MSI doorbell address.

Yes. The KVM code on ARM needs to translate the MSI location from
gIOVA to gPA, because MSI on ARM is behind IOMMU.

> Nicolin, you seems to suggest we could avoid this switching and always return
> System AS. Does that mean we handle this KVM/MSI case separately?
> Could you please detail out the idea?

We could add one of following ops:
    get_msi_address_space
    get_msi_address/translate_msi_iova

Thanks
Nicolin