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

[Liu, Yi L]

> From: Tian, Kevin <kevin.tian@xxxxxxxxx>
> Sent: Wednesday, June 21, 2023 2:02 PM
> 
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Tuesday, June 20, 2023 8:47 PM
> >
> > 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.

Got it.

> 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.

Shall we add it now? I suppose yes.

> >
> > > Intel VT-d supports 4 configurations:
> > >   - passthrough (i.e. identity mapped)
> > >   - S1 only
> > >   - S2 only
> > >   - nested
> > >
> > > 'S2 only' is used when vIOMMU is configured in passthrough.
> >
> > S2 only is modeled as attaching an S2 format iommu domain to the RID,
> > and when this is done the IOAS should gain the reserved regions
> > because it is no different behavior than attaching any other iommu
> > domain to a RID.
> >
> > When the S2 is replaced with a S1 nest then the IOAS should loose
> > those reserved regions since it is no longer attached to a RID.
> 
> yes

Makes sense.

Regards,
Yi Liu

> 
> >
> > > My understanding of ARM SMMU is that from host p.o.v. the CD is the
> > > S1 in the nested configuration. 'identity' is one configuration in the CD
> > > then it's in the business of nesting.
> >
> > I think it is the same. A CD doesn't come into the picture until the
> > guest installs a CD pointing STE. Until that time the S2 is being used
> > as identity.
> >
> > It sounds like the same basic flow.
> 
> After a CD table is installed in a STE I assume the SMMU still allows to
> configure an individual CD entry as identity? e.g. while vSVA is enabled
> on a device the guest can continue to keep CD#0 as identity when the
> default domain of the device is set as 'passthrough'. In this case the
> IOAS still needs to gain reserved regions even though S2 is not directly
> attached from host p.o.v.
> 
> >
> > > My preference was that ALLOC_HWPT allows vIOMMU to opt whether
> > > reserved regions of dev_id should be added to the IOAS of the parent
> > > S2 hwpt.
> >
> > Having an API to explicitly load reserved regions of a specific device
> > to an IOAS makes some sense to me.
> >
> > Jason