Re: [Intel-gfx] [PATCH v3 02/12] vfio/pci: Only check ownership of opened devices in hot reset

["Liu, Yi L" <yi.l.liu@intel.com>]

> From: Eric Auger <eric.auger@redhat.com>
> Sent: Tuesday, April 4, 2023 11:19 PM
> 
> Hi Yi,
> 
> On 4/4/23 16:37, Liu, Yi L wrote:
> > Hi Eric,
> >
> >> From: Eric Auger <eric.auger@redhat.com>
> >> Sent: Tuesday, April 4, 2023 10:00 PM
> >>
> >> Hi YI,
> >>
> >> On 4/1/23 16:44, Yi Liu wrote:
> >>> If the affected device is not opened by any user, it's safe to reset it
> >>> given it's not in use.
> >>>
> >>> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> >>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> >>> Tested-by: Yanting Jiang <yanting.jiang@intel.com>
> >>> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> >>> ---
> >>>  drivers/vfio/pci/vfio_pci_core.c | 14 +++++++++++---
> >>>  include/uapi/linux/vfio.h        |  8 ++++++++
> >>>  2 files changed, 19 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> >>> index 65bbef562268..5d745c9abf05 100644
> >>> --- a/drivers/vfio/pci/vfio_pci_core.c
> >>> +++ b/drivers/vfio/pci/vfio_pci_core.c
> >>> @@ -2429,10 +2429,18 @@ static int vfio_pci_dev_set_hot_reset(struct
> >> vfio_device_set *dev_set,
> >>>  	list_for_each_entry(cur_vma, &dev_set->device_list, vdev.dev_set_list) {
> >>>  		/*
> >>> -		 * Test whether all the affected devices are contained by the
> >>> -		 * set of groups provided by the user.
> >>> +		 * Test whether all the affected devices can be reset by the
> >>> +		 * user.
> >>> +		 *
> >>> +		 * Resetting an unused device (not opened) is safe, because
> >>> +		 * dev_set->lock is held in hot reset path so this device
> >>> +		 * cannot race being opened by another user simultaneously.
> >>> +		 *
> >>> +		 * Otherwise all opened devices in the dev_set must be
> >>> +		 * contained by the set of groups provided by the user.
> >>>  		 */
> >>> -		if (!vfio_dev_in_groups(cur_vma, groups)) {
> >>> +		if (cur_vma->vdev.open_count &&
> >>> +		    !vfio_dev_in_groups(cur_vma, groups)) {
> >>>  			ret = -EINVAL;
> >>>  			goto err_undo;
> >>>  		}
> >>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >>> index 0552e8dcf0cb..f96e5689cffc 100644
> >>> --- a/include/uapi/linux/vfio.h
> >>> +++ b/include/uapi/linux/vfio.h
> >>> @@ -673,6 +673,14 @@ struct vfio_pci_hot_reset_info {
> >>>   * 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 can be affected in the reset
> >> by the reset
> >>> + * while some might be opened by another user.  To avoid interference
> >> s/interference/hot reset failure?
> > I don’t think user can really avoid hot reset failure since there may
> > be new devices plugged into the affected slot. Even user has opened
> I don't know the legacy wrt that issue but this sounds a serious issue,
> meaning the reset of an assigned device could impact another device
> belonging to another group not not owned by the user?

but the hot reset shall fail as the group is not owned by the user.

> > all the groups/devices reported by VFIO_DEVICE_GET_PCI_HOT_RESET_INFO,
> > the hot reset can fail if new device is plugged in and has not been
> > bound to vfio or opened by another user during the window of
> > _INFO and HOT_RESET.
> with respect to the latter isn't the dev_set lock held during the hot
> reset and sufficient to prevent any new opening to occur?

yes. new open needs to acquire the dev_set lock. So when hot reset
acquires the dev_set lock, then no new open can occur. 

Regards,
Yi Liu

> >
> > maybe the whole statement should be as below:
> >
> > To avoid interference, the hot reset can only be conducted when all
> > the affected devices are either opened by the calling user or not
> > opened yet at the moment of the hot reset attempt.
> 
> OK
> 
> Eric
> >
> >>> + * the calling user must ensure all affected devices, if opened, are
> >>> + * owned by itself.
> >>> + *
> >>> + * The ownership is proved by an array of group fds.
> >>> + *
> >>>   * Return: 0 on success, -errno on failure.
> >>>   */
> >>>  struct vfio_pci_hot_reset {
> > Regards,
> > Yi Liu