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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Mar 02, 2023 at 09:55:46AM +0000, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@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.

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.

Jason