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

[Tian, Kevin]

> From: Liu, Yi L <yi.l.liu at intel.com>
> Sent: Wednesday, March 8, 2023 3:47 PM
> 
> > From: Tian, Kevin <kevin.tian at intel.com>
> > Sent: Wednesday, March 8, 2023 3:26 PM
> >
> > > From: Liu, Yi L <yi.l.liu at 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.

I didn't get it. Do we have a iommufd_ctx created but marked as
invalid?

> 
> > 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];
> 	};
> };
> 

why union? Just renaming should work. In the kernel we will first
check whether it's group, whether it's device, then compare
iommufd_ctx is zero length.