On 2022/6/22 20:04, Robin Murphy wrote:
Since IOMMU groups are mandatory for drivers to support, it stands to
reason that any device which has been successfully be added to a group
must be on a bus supported by that IOMMU driver, and therefore a domain
viable for any device in the group must be viable for all devices in
the group. This already has to be the case for the IOMMU API's internal
default domain, for instance. Thus even if the group contains devices on
different buses, that can only mean that the IOMMU driver actually
supports such an odd topology, and so without loss of generality we can
expect the bus type of any device in a group to be suitable for IOMMU
API calls.

Ideally we could remove bus->iommu_ops and all IOMMU APIs go through the

Replace vfio_bus_type() with a simple call to resolve an appropriate
member device from which to then derive a bus type. This is also a step
towards removing the vague bus-based interfaces from the IOMMU API, when
we can subsequently switch to using this device directly.

Furthermore, scrutiny reveals a lack of protection for the bus being
removed while vfio_iommu_type1_attach_group() is using it; the reference
that VFIO holds on the iommu_group ensures that data remains valid, but
does not prevent the group's membership changing underfoot. Holding the
vfio_device for as long as we need here also neatly solves this.

Signed-off-by: Robin Murphy <robin.mur...@arm.com>

Reviewed-by: Lu Baolu <baolu...@linux.intel.com>

Best regards,


After sleeping on it, I decided to type up the helper function approach
to see how it looked in practice, and in doing so realised that with one
more tweak it could also subsume the locking out of the common paths as
well, so end up being a self-contained way for type1 to take care of its
own concern, which I rather like.

  drivers/vfio/vfio.c             | 18 +++++++++++++++++-
  drivers/vfio/vfio.h             |  3 +++
  drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++-------------------
  3 files changed, 31 insertions(+), 20 deletions(-)

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index 61e71c1154be..73bab04880d0 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -448,7 +448,7 @@ static void vfio_group_get(struct vfio_group *group)
   * Device objects - create, release, get, put, search
  /* Device reference always implies a group reference */
-static void vfio_device_put(struct vfio_device *device)
+void vfio_device_put(struct vfio_device *device)
        if (refcount_dec_and_test(&device->refcount))
@@ -475,6 +475,22 @@ static struct vfio_device *vfio_group_get_device(struct 
vfio_group *group,
        return NULL;
+struct vfio_device *vfio_device_get_from_iommu(struct iommu_group *iommu_group)
+       struct vfio_group *group = vfio_group_get_from_iommu(iommu_group);
+       struct vfio_device *device;
+       mutex_lock(&group->device_lock);
+       list_for_each_entry(device, &group->device_list, group_next) {
+               if (vfio_device_try_get(device)) {
+                       mutex_unlock(&group->device_lock);
+                       return device;
+               }
+       }
+       mutex_unlock(&group->device_lock);
+       return NULL;
   * VFIO driver API
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index a67130221151..e8f21e64541b 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -70,3 +70,6 @@ struct vfio_iommu_driver_ops {
int vfio_register_iommu_driver(const struct vfio_iommu_driver_ops *ops);
  void vfio_unregister_iommu_driver(const struct vfio_iommu_driver_ops *ops);
+struct vfio_device *vfio_device_get_from_iommu(struct iommu_group 
+void vfio_device_put(struct vfio_device *device);
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index c13b9290e357..e38b8bfde677 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -1679,18 +1679,6 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
        return ret;
-static int vfio_bus_type(struct device *dev, void *data)
-       struct bus_type **bus = data;
-       if (*bus && *bus != dev->bus)
-               return -EINVAL;
-       *bus = dev->bus;
-       return 0;
  static int vfio_iommu_replay(struct vfio_iommu *iommu,
                             struct vfio_domain *domain)
@@ -2159,7 +2147,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
        struct vfio_iommu *iommu = iommu_data;
        struct vfio_iommu_group *group;
        struct vfio_domain *domain, *d;
-       struct bus_type *bus = NULL;
+       struct vfio_device *iommu_api_dev;
        bool resv_msi, msi_remap;
        phys_addr_t resv_msi_base = 0;
        struct iommu_domain_geometry *geo;
@@ -2192,18 +2180,19 @@ static int vfio_iommu_type1_attach_group(void 
                goto out_unlock;
- /* Determine bus_type in order to allocate a domain */
-       ret = iommu_group_for_each_dev(iommu_group, &bus, vfio_bus_type);
-       if (ret)
+       /* Resolve the group back to a member device for IOMMU API ops */
+       ret = -ENODEV;
+       iommu_api_dev = vfio_device_get_from_iommu(iommu_group);
+       if (!iommu_api_dev)
                goto out_free_group;
ret = -ENOMEM;
        domain = kzalloc(sizeof(*domain), GFP_KERNEL);
        if (!domain)
-               goto out_free_group;
+               goto out_put_dev;
ret = -EIO;
-       domain->domain = iommu_domain_alloc(bus);
+       domain->domain = iommu_domain_alloc(iommu_api_dev->dev->bus);
        if (!domain->domain)
                goto out_free_domain;
@@ -2258,7 +2247,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
        list_add(&group->next, &domain->group_list);
msi_remap = irq_domain_check_msi_remap() ||
-                   iommu_capable(bus, IOMMU_CAP_INTR_REMAP);
+                   iommu_capable(iommu_api_dev->dev->bus, 
if (!allow_unsafe_interrupts && !msi_remap) {
                pr_warn("%s: No interrupt remapping support.  Use the module param 
\"allow_unsafe_interrupts\" to enable VFIO IOMMU support on this platform\n",
@@ -2331,6 +2320,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
+       vfio_device_put(iommu_api_dev);
return 0; @@ -2342,6 +2332,8 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
+       vfio_device_put(iommu_api_dev);

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