RE: [PATCH v5 09/19] vfio/pci: Allow passing zero-length fd array in VFIO_DEVICE_PCI_HOT_RESET

Liu, Yi L
2023-03-08T07:47:39Z

> From: Tian, Kevin <kevin.tian@intel.com>
> Sent: Wednesday, March 8, 2023 3:26 PM
>
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Tuesday, March 7, 2023 9:29 PM
> >
> > >
> > > I really prefer the 'use the iommufd option' still exist, it is so
> > > much cleaner and easier for the actual users of this API. We've lost
> > > the point by worrying about no iommu.
> >
> > Hmmm, so you are suggesting to have both the device fd approach
> > and the zero-length array approach, let user to select the best way
> > based on their wisdom. Is it? how about something like below in the
> > uapi header.
> >
> > /**
> > * VFIO_DEVICE_PCI_HOT_RESET - _IOW(VFIO_TYPE, VFIO_BASE + 13,
> > * struct vfio_pci_hot_reset)
> > *
> > * Userspace requests hot reset for the devices it uses. Due to the
> > * underlying topology, multiple devices may be affected in the reset.
> > * The affected devices may have been opened by the user or by other
> > * users or not opened yet. Only when all the affected devices are
> > * either opened by the current user or not opened by any user, should
> > * the reset request be allowed. Otherwise, this request is expected
> > * to return error. group_fds array can accept either group fds or
> > * device fds. Users using iommufd (valid fd), could also passing a
> > * zero-length group_fds array to indicate using the bound iommufd_ctx
> > * for ownership check to the affected devices that are opened.
> > *
> > * Return: 0 on success, -errno on failure.
> > */
> > struct vfio_pci_hot_reset {
> > __u32 argsz;
> > __u32 flags;
> > __u32 count;
> > __s32 group_fds[];
> > };
> >
>
> * Userspace requests hot reset for the devices it uses. Due to the
> * underlying topology, multiple devices can be affected in the reset
> * while some might be opened by another user. To avoid interference
> * the calling user must ensure all affected devices, if opened, are
> * owned by itself.
> *
> * The ownership can be proved in three ways:
> * - An array of group fds
> * - An array of device fds
> * - A zero-length array
> *
Thanks.
> * In the last case all affected devices which are opened by this user must
> * have been bound to a same iommufd_ctx.

I think we only allow it when this iommufd_ctx is valid. Is it? To
user, it means device should be bound to a positive iommufd.

> and with this change let's rename 'group_fds' to 'fds'

Sure. It would be something like below:

struct vfio_pci_hot_reset {
__u32 argsz;
__u32 flags;
_u32 count;
union {
__s32 group_fds[0];
__s32 fds[0];
};
};

Regards,
Yi Liu