Re: arm64 iommu groups issue

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Mon, Feb 17, 2020 at 12:08:48PM +0000, John Garry wrote:
> > > 
> > > Right, and even worse is that it relies on the port driver even
> > > existing at all.
> > > 
> > > All this iommu group assignment should be taken outside device
> > > driver probe paths.
> > > 
> > > However we could still consider device links for sync'ing the SMMU
> > > and each device probing.
> > 
> > Yes, we should get that for DT now thanks to the of_devlink stuff, but
> > cooking up some equivalent for IORT might be worthwhile.
> 
> It doesn't solve this problem, but at least we could remove the iommu_ops
> check in iort_iommu_xlate().
> 
> We would need to carve out a path from pci_device_add() or even device_add()
> to solve all cases.
> 
> > 
> > > > Another thought that crosses my mind is that when pci_device_group()
> > > > walks up to the point of ACS isolation and doesn't find an existing
> > > > group, it can still infer that everything it walked past *should* be put
> > > > in the same group it's then eventually going to return. Unfortunately I
> > > > can't see an obvious way for it to act on that knowledge, though, since
> > > > recursive iommu_probe_device() is unlikely to end well.
> > > 
> 
> [...]
> 
> > > And this looks to be the reason for which current
> > > iommu_bus_init()->bus_for_each_device(..., add_iommu_group) fails
> > > also.
> > 
> > Of course, just adding a 'correct' add_device replay without the
> > of_xlate process doesn't help at all. No wonder this looked suspiciously
> > simpler than where the first idea left off...
> > 
> > (on reflection, the core of this idea seems to be recycling the existing
> > iommu_bus_init walk rather than building up a separate "waiting list",
> > while forgetting that that wasn't the difficult part of the original
> > idea anyway)
> 
> We could still use a bus walk to add the group per iommu, but we would need
> an additional check to ensure the device is associated with the IOMMU.
> 
> > 
> > > On this current code mentioned, the principle of this seems wrong to
> > > me - we call bus_for_each_device(..., add_iommu_group) for the first
> > > SMMU in the system which probes, but we attempt to add_iommu_group()
> > > for all devices on the bus, even though the SMMU for that device may
> > > yet to have probed.
> > 
> > Yes, iommu_bus_init() is one of the places still holding a
> > deeply-ingrained assumption that the ops go live for all IOMMU instances
> > at once, which is what warranted the further replay in
> > of_iommu_configure() originally. Moving that out of
> > of_platform_device_create() to support probe deferral is where the
> > trouble really started.
> 
> I'm not too familiar with the history here, but could this be reverted now
> with the introduction of of_devlink stuff?

Hi John,

have we managed to reach a consensus on this thread on how to solve
the issue ? Asking because this thread seems stalled - I am keen on
getting it fixed.

Thanks,
Lorenzo
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