Re: [PATCH RFC v3 02/12] iommu: Add a flag to indicate immutable singleton group

[Lu Baolu]

On 2022/4/13 7:32, Tian, Kevin wrote:
> > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Tuesday, April 12, 2022 9:03 PM
> >
> > On 2022/4/12 15:37, Tian, Kevin wrote:
> > > > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > Sent: Tuesday, April 12, 2022 1:09 PM
> > > > On 2022/4/12 11:15, Tian, Kevin wrote:
> > > > > > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > > > Sent: Sunday, April 10, 2022 6:25 PM
> > > > >
> > > > > > This adds a flag in the iommu_group struct to indicate an immutable
> > > > > > singleton group, and uses standard PCI bus topology, isolation features,
> > > > > > and DMA alias quirks to set the flag. If the device came from DT,
> > assume
> > > > > > it is static and then the singleton attribute can know from the device
> > > > > > count in the group.
> > > > >
> > > > > where does the assumption come from?
> > > >
> > > > Hotplug is the only factor that can dynamically affect the
> > > > characteristics of IOMMU group singleton as far as I can see. If a
> > > > device node was created from the DT, it could be treated as static,
> > > > hence we can judge the singleton in iommu probe phase during boot.
> > >
> > > I didn't get this. Let's look at your code in iommu_group_add_device():
> > >
> > >    +	else if (is_of_node(dev_fwnode(dev)))
> > >    +		group->immutable_singleton =
> > >    +				(iommu_group_device_count(group) == 1);
> > >
> > > Even if there is a multi-devices group above logic will set the flag when
> > > the first device in the group is added since at that time there is only
> > > one device in the group. We need other concrete information  to tell
> > > it similar to how you walk PCI hierarchy to find out the fact...
> >
> > This is a small trick to make things simpler. Once more devices are
> > added to the group, the flag will be flipped. All iommu_group's should
> > be settled down before any drivers start to consume this flag.
>
> As an immutable flag it cannot be flipped. What about SVA has been
> enabled on the 1st device before the 2nd one is added to the group?

The flipping happens during iommu probe phase before any device driver
binding. Anyway, I have to agree that it doesn't look clever. :-) Let me
try to figure out another way.

Best regards,
baolu