Re: [PATCH v2 00/11] iommufd: Add nesting infrastructure

[Nicolin Chen <nicolinc@nvidia.com>]

On Wed, Jun 21, 2023 at 06:02:21AM +0000, Tian, Kevin wrote:

> > On Tue, Jun 20, 2023 at 01:43:42AM +0000, Tian, Kevin wrote:
> > > I wonder whether we have argued passed each other.
> > >
> > > This series adds reserved regions to S2. I challenged the necessity as
> > > S2 is not directly accessed by the device.
> > >
> > > Then you replied that doing so still made sense to support identity
> > > S1.
> >
> > I think I said/ment if we attach the "s2" iommu domain as a direct
> > attach for identity - eg at boot time, then the IOAS must gain the
> > reserved regions. This is our normal protocol.
> >
> > But when we use the "s2" iommu domain as an actual nested S2 then we
> > don't gain reserved regions.
> 
> Then we're aligned.
> 
> Yi/Nicolin, please update this series to not automatically add reserved
> regions to S2 in the nesting configuration.

I'm a bit late for the conversation here. Yet, how about the
IOMMU_RESV_SW_MSI on ARM in the nesting configuration? We'd
still call iommufd_group_setup_msi() on the S2 HWPT, despite
attaching the device to a nested S1 HWPT right?

> It also implies that the user cannot rely on IOAS_IOVA_RANGES to
> learn reserved regions for arranging addresses in S1.
> 
> Then we also need a new ioctl to report reserved regions per dev_id.

So, in a nesting configuration, QEMU would poll a device's S2
MSI region (i.e. IOMMU_RESV_SW_MSI) to prevent conflict?

Thanks
Nic