On 06/21/2013 12:55 AM, Alex Williamson wrote: > On Thu, 2013-06-20 at 18:48 +1000, Alexey Kardashevskiy wrote: >> On 06/20/2013 05:47 PM, Benjamin Herrenschmidt wrote: >>> On Thu, 2013-06-20 at 15:28 +1000, David Gibson wrote: >>>>> Just out of curiosity - would not get_file() and fput_atomic() on a >>>> group's >>>>> file* do the right job instead of vfio_group_add_external_user() and >>>>> vfio_group_del_external_user()? >>>> >>>> I was thinking that too. Grabbing a file reference would certainly be >>>> the usual way of handling this sort of thing. >>> >>> But that wouldn't prevent the group ownership to be returned to >>> the kernel or another user would it ? >> >> >> Holding the file pointer does not let the group->container_users counter go >> to zero > > How so? Holding the file pointer means the file won't go away, which > means the group release function won't be called. That means the group > won't go away, but that doesn't mean it's attached to an IOMMU. A user > could call UNSET_CONTAINER. > >> and this is exactly what vfio_group_add_external_user() and >> vfio_group_del_external_user() do. The difference is only in absolute value >> - 2 vs. 3. >> >> No change in behaviour whether I use new vfio API or simply hold file* till >> KVM closes fd created when IOMMU was connected to LIOBN. > > By that notion you could open(/dev/vfio/$GROUP) and you're safe, right? > But what about SET_CONTAINER & SET_IOMMU? All that you guarantee > holding the file pointer is that the vfio_group exists. > >> And while this counter is not zero, QEMU cannot take ownership over the >> group. >> >> I am definitely still missing the bigger picture... > > The bigger picture is that the group needs to exist AND it needs to be > setup and maintained to have IOMMU protection. Actually, my first stab > at add_external_user doesn't look sufficient, it needs to look more like > vfio_group_get_device_fd, checking group->container->iommu and > group_viable().
This makes sense. If you did this, that would be great. Without it, I really cannot see how the proposed inc/dec of container_users is better than simple holding file*. Thanks. > As written it would allow an external user after > SET_CONTAINER without SET_IOMMU. It should also be part of the API that > the external user must hold the file reference between add_external_use > and del_external_user and do cleanup on any exit paths. Thanks, -- Alexey -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
