Re: [PATCH v3 07/17] iommufd: Add IOMMU_RESV_IOVA_RANGES

[Jason Gunthorpe]

On Mon, Jul 31, 2023 at 06:14:50AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Saturday, July 29, 2023 1:17 AM
> > 
> > On Fri, Jul 28, 2023 at 10:07:58AM +0000, Tian, Kevin wrote:
> > > > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > > Sent: Monday, July 24, 2023 7:04 PM
> > > >
> > > > This reports device's reserved IOVA regions to userspace. This is needed
> > > > in the nested translation as userspace owns stage-1 HWPT, and
> > userspace
> > > > needs to exclude the reserved IOVA regions in the stage-1 HWPT hence
> > > > exclude
> > > > them in the device's DMA address space.
> > > >
> > > > This can also be used to figure out allowed IOVAs of an IOAS.
> > >
> > > We may need a special type to mark SW_MSI since it requires identity
> > > mapping in stage-1 instead of being reserved.
> > 
> > Only the kernel can do this, so there is no action for user space to
> > take beyond knowing that is is not mappable IOVA.
> > 
> > The merit for "SW_MSI" may be to inform the rest of the system about
> > the IOVA of the ITS page, but with the current situation that isn't
> > required since only the kernel needs that information.
> 
> IIUC guest kernel needs to know the "SW_MSI" region and then setup an
> 1:1 mapping for it in S1.

Yes, but qemu hardcodes this and for some reason people thought that
was a good idea back when.

> > I think the long term way forward is to somehow arrange for the SW_MSI
> > to not become mapped when creating the parent HWPT and instead cause
> > the ITS page to be mapped through some explicit IOCTL.
> 
> yes this is a cleaner approach. Qemu selects the intermediate address of
> vITS page and maps it to physical ITS page in S2. Then the guest kernel
> just pick whatever "SW_MSI" address in S1 to vITS as it does today on 
> bare metal.

Right, so I've been inclined to minimize the amount of special stuff
created for this way of doing the MSI and hope we can reach a better
way sooner than later

Jason