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

[Liu, Yi L]

> From: Jason Gunthorpe <jgg at nvidia.com>
> Sent: Thursday, March 2, 2023 8:35 PM
> 
> On Thu, Mar 02, 2023 at 09:55:46AM +0000, Tian, Kevin wrote:
> > > From: Liu, Yi L <yi.l.liu at intel.com>
> > > Sent: Thursday, March 2, 2023 2:07 PM
> > >
> > > > -		if (!vfio_dev_in_groups(cur_vma, groups)) {
> > > > +		if (cur_vma->vdev.open_count &&
> > > > +		    !vfio_dev_in_groups(cur_vma, groups) &&
> > > > +		    !vfio_dev_in_iommufd_ctx(cur_vma, iommufd_ctx)) {
> > >
> > > Hi Alex, Jason,
> > >
> > > There is one concern on this approach which is related to the
> > > cdev noiommu mode. As patch 16 of this series, cdev path
> > > supports noiommu mode by passing a negative iommufd to
> > > kernel. In such case, the vfio_device is not bound to a valid
> > > iommufd. Then the check in vfio_dev_in_iommufd_ctx() is
> > > to be broken.
> > >
> > > An idea is to add a cdev_noiommu flag in vfio_device, when
> > > checking the iommufd_ictx, also check this flag. If all the opened
> > > devices in the dev_set have vfio_device->cdev_noiommu==true,
> > > then the reset is considered to be doable. But there is a special
> > > case. If devices in this dev_set are opened by two applications
> > > that operates in cdev noiommu mode, then this logic is not able
> > > to differentiate them. In that case, should we allow the reset?
> > > It seems to ok to allow reset since noiommu mode itself means
> > > no security between the applications that use it. thoughts?
> > >
> >
> > Probably we need still pass in a valid iommufd (instead of using
> > a negative value) in noiommu case to mark the ownership so the
> > check in the reset path can correctly catch whether an opened
> > device belongs to this user.
> 
> There should be no iommufd at all in no-iommu mode
> 
> Adding one just to deal with noiommu reset seems pretty sad :\
> 
> no-iommu is only really used by dpdk, and it doesn't invoke
> VFIO_DEVICE_PCI_HOT_RESET at all.

Does it happen to be or by design, this ioctl is not needed by dpdk?

> I'd say as long as VFIO_DEVICE_PCI_HOT_RESET works if only one vfio
> device is open using a empty list (eg we should ensure that the
> invoking cdev itself is allowed) then I think it is OK.

Sorry, which empty list are your referring?

Regards,
Yi Liu