Re: arm64 iommu groups issue

[Robin Murphy]

On 13/02/2020 3:49 pm, John Garry wrote:
> > > The underlying issue is that, for historical reasons, OF/IORT-based
> > > IOMMU drivers have ended up with group allocation being tied to endpoint
> > > driver probing via the dma_configure() mechanism (long story short,
> > > driver probe is the only thing which can be delayed in order to wait for
> > > a specific IOMMU instance to be ready).However, in the meantime, the
> > > IOMMU API internals have evolved sufficiently that I think there's a way
> > > to really put things right - I have the spark of an idea which I'll try
> > > to sketch out ASAP...
> >
> > OK, great.
>
> Hi Robin,
>
> I was wondering if you have had a chance to consider this problem again?

Yeah, sorry, more things came up such that it still hasn't been P yet ;)

Lorenzo and I did get as far as discussing it a bit more and writing up 
a ticket, so it's formally on our internal radar now, at least.

> One simple idea could be to introduce a device link between the endpoint 
> device and its parent bridge to ensure that they probe in order, as 
> expected in pci_device_group():
>
> Subject: [PATCH] PCI: Add device link to ensure endpoint device driver 
> probes after bridge
>
> It is required to ensure that a device driver for an endpoint will probe
> after the parent port driver, so add a device link for this.
>
> ---
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 512cb4312ddd..4b832ad25b20 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2383,6 +2383,7 @@ static void pci_set_msi_domain(struct pci_dev *dev)
>  Âvoid pci_device_add(struct pci_dev *dev, struct pci_bus *bus)
>  Â{
>  ÂÂÂÂ int ret;
> +ÂÂÂ struct device *parent;
>
>  ÂÂÂÂ pci_configure_device(dev);
>
> @@ -2420,6 +2421,10 @@ void pci_device_add(struct pci_dev *dev, struct 
> pci_bus *bus)
>  ÂÂÂÂ /* Set up MSI IRQ domain */
>  ÂÂÂÂ pci_set_msi_domain(dev);
>
> +ÂÂÂ parent = dev->dev.parent;
> +ÂÂÂ if (parent && parent->bus == &pci_bus_type)
> +ÂÂÂÂÂÂÂ device_link_add(&dev->dev, parent, DL_FLAG_AUTOPROBE_CONSUMER);
> +
>  ÂÂÂÂ /* Notifier could use PCI capabilities */
>  ÂÂÂÂ dev->match_driver = false;
>  ÂÂÂÂ ret = device_add(&dev->dev);
> --
>
> This would work, but the problem is that if the port driver fails in 
> probing - and not just for -EPROBE_DEFER - then the child device will 
> never probe. This very thing happens on my dev board. However we could 
> expand the device links API to cover this sort of scenario.

Yes, that's an undesirable issue, but in fact I think it's mostly 
indicative that involving drivers in something which is designed to 
happen at a level below drivers is still fundamentally wrong and doomed 
to be fragile at best.

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.

> As for alternatives, it looks pretty difficult to me to disassociate the 
> group allocation from the dma_configure path.

Indeed it's non-trivial, but it really does need cleaning up at some point.

Having just had yet another spark, does something like the untested 
super-hack below work at all? I doubt it's a viable direction to take in 
itself, but it could be food for thought if it at least proves the concept.

Robin.

----->8-----
diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index aa3ac2a03807..554cde76c766 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -3841,20 +3841,20 @@ static int arm_smmu_set_bus_ops(struct iommu_ops 
*ops)
 	int err;

 #ifdef CONFIG_PCI
-	if (pci_bus_type.iommu_ops != ops) {
+	if (1) {
 		err = bus_set_iommu(&pci_bus_type, ops);
 		if (err)
 			return err;
 	}
 #endif
 #ifdef CONFIG_ARM_AMBA
-	if (amba_bustype.iommu_ops != ops) {
+	if (1) {
 		err = bus_set_iommu(&amba_bustype, ops);
 		if (err)
 			goto err_reset_pci_ops;
 	}
 #endif
-	if (platform_bus_type.iommu_ops != ops) {
+	if (1) {
 		err = bus_set_iommu(&platform_bus_type, ops);
 		if (err)
 			goto err_reset_amba_ops;
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 660eea8d1d2f..b81ae2b4d4fb 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1542,6 +1542,14 @@ static int iommu_bus_init(struct bus_type *bus, 
const struct iommu_ops *ops)
 	return err;
 }

+static int iommu_bus_replay(struct device *dev, void *data)
+{
+	if (dev->iommu_group)
+		return 0;
+
+	return add_iommu_group(dev, data);
+}
+
 /**
  * bus_set_iommu - set iommu-callbacks for the bus
  * @bus: bus.
@@ -1564,6 +1572,9 @@ int bus_set_iommu(struct bus_type *bus, const 
struct iommu_ops *ops)
 		return 0;
 	}

+	if (bus->iommu_ops == ops)
+		return bus_for_each_dev(bus, NULL, NULL, iommu_bus_replay);
+
 	if (bus->iommu_ops != NULL)
 		return -EBUSY;