Messages in this thread | | | Date | Tue, 21 Jun 2022 20:09:20 +0100 | Subject | Re: [PATCH 1/2] vfio/type1: Simplify bus_type determination | From | Robin Murphy <> |
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On 2022-06-10 01:03, Jason Gunthorpe via iommu wrote: > On Wed, Jun 08, 2022 at 03:25:49PM +0100, 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 arbitrary device in a group to be suitable >> for IOMMU API calls. >> >> Replace vfio_bus_type() with a trivial callback that simply returns any >> device from which to then derive a usable bus type. This is also a step >> towards removing the vague bus-based interfaces from the IOMMU API. >> >> Furthermore, scrutiny reveals a lack of protection for the bus and/or >> device being removed while .attach_group is inspecting them; the >> reference we hold on the iommu_group ensures that data remains valid, >> but does not prevent the group's membership changing underfoot. Holding >> the vfio_goup's device_lock should be sufficient to block any relevant >> device's VFIO driver from unregistering, and thus block unbinding and >> any further stages of removal for the duration of the attach operation. > > The device_lock only protects devices that are on the device_list from > concurrent unregistration, the device returned by > iommu_group_for_each_dev() is not guarented to be the on the device > list.
Sigh, you're quite right, and now I have a vague feeling that you called that out in the previous discussion too, so apologies for forgetting.
>> @@ -760,8 +760,11 @@ static int __vfio_container_attach_groups(struct vfio_container *container, >> int ret = -ENODEV; >> >> list_for_each_entry(group, &container->group_list, container_next) { >> + /* Prevent devices unregistering during attach */ >> + mutex_lock(&group->device_lock); >> ret = driver->ops->attach_group(data, group->iommu_group, >> group->type); >> + mutex_unlock(&group->device_lock); > > I still prefer the version where we pass in an arbitrary vfio_device > from the list the group maintains: > > list_first_entry(group->device_list) > > And don't call iommu_group_for_each_dev(), it is much simpler to > reason about how it works.
Agreed, trying to figure out which are the VFIO devices from within the iommu_group iterator seems beyond the threshold of practicality.
Quick consensus then: does anyone have a particular preference between changing the .attach_group signature vs. adding a helper based on vfio_group_get_from_iommu() for type1 to call from within its callback? They seem about equal (but opposite) in terms of the simplicity vs. impact tradeoff to me, so I can't quite decide conclusively...
Thanks, Robin.
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