Hi Yi, On 4/7/23 05:42, Liu, Yi L wrote: >> From: Alex Williamson <alex.williamson@xxxxxxxxxx> >> Sent: Friday, April 7, 2023 2:58 AM >>>> You don't say anything about potential restriction, ie. what if the user calls >>>> KVM_DEV_VFIO_FILE with device fds while it has been using legacy >> container/group >>>> API? >>> legacy container/group path cannot do it as the below enhancement. >>> User needs to call KVM_DEV_VFIO_FILE before open devices, so this >>> should happen before _GET_DEVICE_FD. So the legacy path can never >>> pass device fds in KVM_DEV_VFIO_FILE. >>> >>> >> https://lore.kernel.org/kvm/20230327102059.333d6976.alex.williamson@xxxxxxxxxx >> /#t >> >> Wait, are you suggesting that a comment in the documentation suggesting >> a usage policy somehow provides enforcement of that ordering?? That's >> not how this works. Thanks, > I don't know if there is a good way to enforce this order in the code. The > vfio_device->kvm pointer is optional. If it is NULL, vfio just ignores it. > So vfio doesn't have a good way to tell if the order requirement is met or > not. Perhaps just trigger NULL pointer dereference when kvm pointer is used > in the device drivers like kvmgt if this order is not met. > > So that's why I come up to document it here. The applications uses kvm > should know this and follow this otherwise it may encounter error. > > Do you have other suggestions for it? This order should be a generic > requirement. is it? group path also needs to follow it to make the mdev > driver that refers kvm pointer to be workable. In the same way as kvm_vfio_file_is_valid() called in kvm_vfio_file_add() can't you have a kernel API that checks the fd consistence? Thanks Eric > > Thanks, > Yi Liu > >>>>> -The GROUP_ADD operation above should be invoked prior to accessing the >>>>> +The FILE/GROUP_ADD operation above should be invoked prior to accessing the >>>>> device file descriptor via VFIO_GROUP_GET_DEVICE_FD in order to support >>>>> drivers which require a kvm pointer to be set in their .open_device() >>>>> -callback. >>>>> +callback. It is the same for device file descriptor via character device >>>>> +open which gets device access via VFIO_DEVICE_BIND_IOMMUFD. For such file >>>>> +descriptors, FILE_ADD should be invoked before >> VFIO_DEVICE_BIND_IOMMUFD >>>>> +to support the drivers mentioned in prior sentence as well. >>> just as here. This means device fds can only be passed with KVM_DEV_VFIO_FILE >>> in the cdev path. >>> >>> Regards, >>> Yi Liu
