Re: [PATCH v3 12/12] vfio/pci: Report dev_id in VFIO_DEVICE_GET_PCI_HOT_RESET_INFO

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Apr 14, 2023 at 09:11:30AM +0000, Tian, Kevin wrote:

> The only corner case with this option is when a user mixes group
> and cdev usages. iirc you mentioned it's a valid usage to be supported.
> In that case the kernel doesn't have sufficient knowledge to judge
> 'resettable' as it doesn't know which groups are opened by this user.

IMHO we don't need to support this combination.

We can say that to use the hot reset API the user must put all their
devices into the same iommufd_ctx and cover 100% of the known use
cases for this.

There are already other situations, like nesting, that do force users
to put everything into one iommufd_ctx.

No reason to make things harder and more complicated.

I'm coming to the feeling that we should put no-iommu devices in
iommufd_ctx's as well. They would be an iommufd_access like
mdevs. That would clean up the complications they cause here.

I suppose we should have done that from the beginning - no-iommu is an
IOMMUFD access, it just uses a crazy /proc based way to learn the
PFNs. Making it a proper access and making a real VFIO ioctl that
calls iommufd_access_pin_pages() and returns the DMA mapped addresses
to userspace would go a long way to making no-iommu work in a logical,
usable, way.

Jason