Re: [PATCH v1 1/2] vfio: add callback to get tph info for dma-buf

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Apr 22, 2026 at 09:23:27AM -0600, Alex Williamson wrote:
> In general though, I'm really hoping that someone interested in
> enabling TPH as an interface through vfio actually decides to take
> resource targeting and revocation seriously.  There's no validation of
> the steering tag here relative to what the user has access to and no
> mechanism to revoke those tags if access changes.  In fact, there's not
> even a proposed mechanism allowing the user to derive valid steering
> tags.  Does the user implicitly know the value and the kernel just
> allows it because... yolo? 

This is the steering tag that remote devices will send *INTO* the VFIO
device.

IMHO it is entirely appropriate that the driver controlling the device
decide what tags are sent into it and when, so that's the VFIO
userspace.

There is no concept of access here since the entire device is captured
by VFIO.

If the VFIO device catastrophically malfunctions when receiving
certain steering tags then it is incompatible with VFIO and we should
at least block this new API..

The only requirement is that the device limit the TPH to only the
function that is perceiving them. If a device is really broken and
doesn't meet that then it should be blocked off and it is probably not
safe to be used with VMs at all.

Jason