Re: [PATCH v2 2/3] iommu/dma: Support MSIs through nested domains

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Aug 09, 2024 at 03:41:36PM -0300, Jason Gunthorpe wrote:
> On Thu, Aug 08, 2024 at 01:38:44PM +0100, Robin Murphy wrote:
> > On 06/08/2024 9:25 am, Tian, Kevin wrote:
> > > > From: Nicolin Chen <nicolinc@nvidia.com>
> > > > Sent: Saturday, August 3, 2024 8:32 AM
> > > > 
> > > > From: Robin Murphy <robin.murphy@arm.com>
> > > > 
> > > > Currently, iommu-dma is the only place outside of IOMMUFD and drivers
> > > > which might need to be aware of the stage 2 domain encapsulated within
> > > > a nested domain. This would be in the legacy-VFIO-style case where we're
> > > 
> > > why is it a legacy-VFIO-style? We only support nested in IOMMUFD.
> > 
> > Because with proper nesting we ideally shouldn't need the host-managed MSI
> > mess at all, which all stems from the old VFIO paradigm of completely
> > abstracting interrupts from userspace. I'm still hoping IOMMUFD can grow its
> > own interface for efficient MSI passthrough, where the VMM can simply map
> > the physical MSI doorbell into whatever IPA (GPA) it wants it to appear at
> > in the S2 domain, then whatever the guest does with S1 it can program the
> > MSI address into the endpoint accordingly without us having to fiddle with
> > it.
> 
> +1
> 
> I don't have a staged plan to do this though. Getting the ITS page
> into the S2 at a user specified address should be simple enough to
> manage.
> 
> The bigger issue is that we still have the hypervisor GIC driver
> controlling things and it will need to know to use the guest provided
> MSI address captured during the MSI trap, not its own address. I don't
> have an idea how to connect those two parts yet.

You mean the gPA of the vITS v.s. PA of the ITS, right? I think
that's because only VMM knows the virtual IRQ number to insert?
We don't seem to have a choice for that unless we want to poke
a hole to the vGIC design..

With that, it feels a quite big improvement already by getting
rid of the entire shadow MSI mapping, including msi_cookie and
RMR..

Thanks
Nicolin