Date:   Mon, 27 Sep 2021 09:42:58 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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Subject: RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma
 interfaces

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, September 22, 2021 8:40 PM
> 
> > > Ie the basic flow would see the driver core doing some:
> >
> > Just double confirm. Is there concern on having the driver core to
> > call iommu functions?
> 
> It is always an interesting question, but I'd say iommu is
> foundantional to Linux and if it needs driver core help it shouldn't
> be any different from PM, pinctl, or other subsystems that have
> inserted themselves into the driver core.
> 
> Something kind of like the below.
> 
> If I recall, once it is done like this then the entire iommu notifier
> infrastructure can be ripped out which is a lot of code.

Currently vfio is the only user of this notifier mechanism. Now 
three events are handled in vfio_iommu_group_notifier():

NOTIFY_ADD_DEVICE: this is basically for some sanity check. suppose
not required once we handle it cleanly in the iommu/driver core.

NOTIFY_BOUND_DRIVER: the BUG_ON() logic to be fixed by this change.

NOTIFY_UNBOUND_DRIVER: still needs some thoughts. Based on
the comments the group->unbound_list is used to avoid breaking
group viability check between vfio_unregister_group_dev() and 
final dev/drv teardown. within that small window the device is
not tracked by vfio group but is still bound to a driver (e.g. vfio-pci
itself), while an external group user may hold a reference to the
group. Possibly it's not required now with the new mechanism as 
we rely on init/exit_user_dma() as the single switch to claim/
withdraw the group ownership. As long as exit_user_dma() is not 
called until vfio_group_release(), above small window is covered
thus no need to maintain a unbound_list.

But anyway since this corner case is tricky, will think more in case
of any oversight.

> 
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 68ea1f949daa90..e39612c99c6123 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -566,6 +566,10 @@ static int really_probe(struct device *dev, struct
> device_driver *drv)
>                 goto done;
>         }
> 
> +       ret = iommu_set_kernel_ownership(dev);
> +       if (ret)
> +               return ret;
> +
>  re_probe:
>         dev->driver = drv;
> 
> @@ -673,6 +677,7 @@ static int really_probe(struct device *dev, struct
> device_driver *drv)
>                 dev->pm_domain->dismiss(dev);
>         pm_runtime_reinit(dev);
>         dev_pm_set_driver_flags(dev, 0);
> +       iommu_release_kernel_ownership(dev);
>  done:
>         return ret;
>  }
> @@ -1214,6 +1219,7 @@ static void __device_release_driver(struct device
> *dev, struct device *parent)
>                         dev->pm_domain->dismiss(dev);
>                 pm_runtime_reinit(dev);
>                 dev_pm_set_driver_flags(dev, 0);
> +               iommu_release_kernel_ownership(dev);
> 
>                 klist_remove(&dev->p->knode_driver);
>                 device_pm_check_callbacks(dev);

I expanded above into below conceptual draft. Please help check whether
it matches your thought:

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 68ea1f9..826a651 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -566,6 +566,10 @@ static int really_probe(struct device *dev, struct device_driver *drv)
 		goto done;
 	}
 
+	ret = iommu_device_set_dma_hint(dev, drv->dma_hint);
+	if (ret)
+		return ret;
+
 re_probe:
 	dev->driver = drv;
 
@@ -673,6 +677,7 @@ static int really_probe(struct device *dev, struct device_driver *drv)
 		dev->pm_domain->dismiss(dev);
 	pm_runtime_reinit(dev);
 	dev_pm_set_driver_flags(dev, 0);
+	iommu_device_clear_dma_hint(dev);
 done:
 	return ret;
 }
@@ -1214,6 +1219,7 @@ static void __device_release_driver(struct device *dev, struct device *parent)
 			dev->pm_domain->dismiss(dev);
 		pm_runtime_reinit(dev);
 		dev_pm_set_driver_flags(dev, 0);
+		iommu_device_clear_dma_hint(dev);
 
 		klist_remove(&dev->p->knode_driver);
 		device_pm_check_callbacks(dev);
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 3303d70..b12f335 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1064,6 +1064,104 @@ void iommu_group_put(struct iommu_group *group)
 }
 EXPORT_SYMBOL_GPL(iommu_group_put);
 
+static int iommu_dev_viable(struct device *dev, void *data)
+{
+	enum dma_hint hint = *data;
+	struct device_driver *drv = READ_ONCE(dev->driver);
+
+	/* no conflict if the new device doesn't do DMA */
+	if (hint == DMA_FOR_NONE)
+		return 0;
+
+	/* no conflict if this device is driver-less, or doesn't do DMA */
+	if (!drv || (drv->dma_hint == DMA_FOR_NONE))
+		return 0;
+
+	/* kernel dma and user dma are exclusive */
+	if (hint != drv->dma_hint)
+		return -EINVAL;
+
+	/*
+	 * devices in the group could be bound to different user-dma
+	 * drivers (e.g. vfio-pci, vdpa, etc.), or even bound to the
+	 * same driver but eventually opened via different mechanisms
+	 * (e.g. vfio group vs. nongroup interfaces). We rely on 
+	 * iommu_{group/device}_init_user_dma() to ensure exclusive
+	 * user-dma ownership (iommufd ctx, vfio container ctx, etc.)
+	 * in such scenario.
+	 */
+	return 0;
+}
+
+static int __iommu_group_viable(struct iommu_group *group, enum dma_hint hint)
+{
+	return (__iommu_group_for_each_dev(group, &hint,
+					   iommu_dev_viable) == 0);
+}
+
+int iommu_device_set_dma_hint(struct device *dev, enum dma_hint hint)
+{
+	struct iommu_group *group;
+	int ret;
+
+	group = iommu_group_get(dev);
+	/* not an iommu-probed device */
+	if (!group)
+		return 0;
+
+	mutex_lock(&group->mutex);
+	ret = __iommu_group_viable(group, hint);
+	mutex_unlock(&group->mutex);
+
+	iommu_group_put(group);
+	return ret;
+}
+
+/* any housekeeping? */
+void iommu_device_clear_dma_hint(struct device *dev) {}
+
+/* claim group ownership for user-dma */
+int __iommu_group_init_user_dma(struct iommu_group *group,
+				unsigned long owner)
+{
+	int ret;
+
+	ret = __iommu_group_viable(group, DMA_FOR_USER);
+	if (ret)
+		goto out;
+
+	/* other logic for exclusive user_dma ownership and refcounting */
+out:
+	return ret;
+}
+
+int iommu_group_init_user_dma(struct iommu_group *group,
+			      unsigned long owner)
+{
+	int ret;
+
+	mutex_lock(&group->mutex);
+	ret = __iommu_group_init_user_dma(group, owner);
+	mutex_unlock(&group->mutex);
+	return ret;
+}
+
+int iommu_device_init_user_dma(struct device *dev,
+			      unsigned long owner)
+{
+	struct iommu_group *group = iommu_group_get(dev);
+	int ret;
+
+	if (!group)
+		return -ENODEV;
+
+	mutex_lock(&group->mutex);
+	ret = __iommu_group_init_user_dma(group, owner);
+	mutex_unlock(&group->mutex);
+	iommu_grou_put(group);
+	return ret;
+}
+
 /**
  * iommu_group_register_notifier - Register a notifier for group changes
  * @group: the group to watch
diff --git a/drivers/pci/pci-stub.c b/drivers/pci/pci-stub.c
index e408099..4568811 100644
--- a/drivers/pci/pci-stub.c
+++ b/drivers/pci/pci-stub.c
@@ -36,6 +36,9 @@ static int pci_stub_probe(struct pci_dev *dev, const struct pci_device_id *id)
 	.name		= "pci-stub",
 	.id_table	= NULL,	/* only dynamic id's */
 	.probe		= pci_stub_probe,
+	.driver = {
+		.dma_hint	= DMA_FOR_NONE,
+	},
 };
 
 static int __init pci_stub_init(void)
diff --git a/drivers/vdpa/ifcvf/ifcvf_main.c b/drivers/vdpa/ifcvf/ifcvf_main.c
index dcd648e..a613b78 100644
--- a/drivers/vdpa/ifcvf/ifcvf_main.c
+++ b/drivers/vdpa/ifcvf/ifcvf_main.c
@@ -678,6 +678,9 @@ static void ifcvf_remove(struct pci_dev *pdev)
 	.id_table = ifcvf_pci_ids,
 	.probe    = ifcvf_probe,
 	.remove   = ifcvf_remove,
+	.driver = {
+		.dma_hint	= DMA_FOR_USER,
+	},
 };
 
 module_pci_driver(ifcvf_driver);
diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index a5ce92b..61c422d 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -193,6 +193,9 @@ static int vfio_pci_sriov_configure(struct pci_dev *pdev, int nr_virtfn)
 	.remove			= vfio_pci_remove,
 	.sriov_configure	= vfio_pci_sriov_configure,
 	.err_handler		= &vfio_pci_core_err_handlers,
+	.driver = {
+		.dma_hint	= DMA_FOR_USER,
+	},
 };
 
 static void __init vfio_pci_fill_ids(void)
diff --git a/include/linux/device/driver.h b/include/linux/device/driver.h
index a498ebc..6bddfd2 100644
--- a/include/linux/device/driver.h
+++ b/include/linux/device/driver.h
@@ -48,6 +48,17 @@ enum probe_type {
 };
 
 /**
+ * enum dma_hint - device driver dma hint
+ *	Device drivers may provide hints for whether dma is
+ *	intended for kernel driver, user driver, not not required.
+ */
+enum dma_hint {
+	DMA_FOR_KERNEL,
+	DMA_FOR_USER,
+	DMA_FOR_NONE,
+};
+
+/**
  * struct device_driver - The basic device driver structure
  * @name:	Name of the device driver.
  * @bus:	The bus which the device of this driver belongs to.
@@ -101,6 +112,7 @@ struct device_driver {
 
 	bool suppress_bind_attrs;	/* disables bind/unbind via sysfs */
 	enum probe_type probe_type;
+	enum dma_type dma_type;
 
 	const struct of_device_id	*of_match_table;
 	const struct acpi_device_id	*acpi_match_table;

Thanks
Kevin

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