linux-kernel - Re: arm64 iommu groups issue

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Date:   Mon, 17 Feb 2020 12:08:48 +0000
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>, Marc Zyngier <maz@...nel.org>,
        "Will Deacon" <will@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        "Guohanjun (Hanjun Guo)" <guohanjun@...wei.com>
CC:     iommu <iommu@...ts.linux-foundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linuxarm <linuxarm@...wei.com>,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>,
        "Alex Williamson" <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Saravana Kannan" <saravanak@...gle.com>
Subject: Re: arm64 iommu groups issue

>>
>> 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?

Cheers,
John