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

[Lu Baolu]

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.

> > > > +	/*
> > > > +	 * The device could be considered to be fully isolated if
> > > > +	 * all devices on the path from the parent to the host-PCI
> > > > +	 * bridge are protected from peer-to-peer DMA by ACS.
> > > > +	 */
> > > > +	if (!pci_is_root_bus(pdev->bus) &&
> > > > +	    !pci_acs_path_enabled(pdev->bus->self, NULL, REQ_ACS_FLAGS))
> > > > +		return false;
> > > > +
> > > > +	/* Multi-function devices should have ACS enabled. */
> > > > +	if (pdev->multifunction && !pci_acs_enabled(pdev, REQ_ACS_FLAGS))
> > > > +		return false;
> > >
> > > Looks my earlier comment was lost, i.e. you can just use
> > > pci_acs_path_enabled(pdev) to cover above two checks.
> >
> > If a device is directly connected to the root bridge and it is not an
> > MFD, do we still need ACS on it? The Intel idxd device seems to be such
> > a device. I had a quick check with lspci, it has no ACS support.
> >
> > I probably missed anything.
>
> single-function RCiEP doesn't need to implement ACS but this has
> been covered by pci_acs_enabled() and pci_acs_path_enabled().

Cool! I missed this part. :-) Thanks a lot.

Best regards,
baolu