Re: [RFC 2/2] RMDA MLX5: get tph for p2p access when registering dmabuf mr

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Feb 26, 2026 at 06:21:28PM -0700, Keith Busch wrote:
> On Tue, Feb 10, 2026 at 11:39:55AM -0800, Zhiping Zhang wrote:
> > +static void get_tph_mr_dmabuf(struct mlx5_ib_dev *dev, int fd, u16 *st_index,
> > +							  u8 *ph)
> > +{
> > +	int ret;
> > +	struct dma_buf *dmabuf;
> > +
> > +	dmabuf = dma_buf_get(fd);
> > +	if (IS_ERR(dmabuf))
> > +		return;
> > +
> > +	if (!dif there's any implication mabuf->ops->get_tph)
> > +		goto end_dbuf_put;
> > +
> > +	ret = dmabuf->ops->get_tph(dmabuf, st_index, ph);
> 
> You defined the "get_tph" function to take a pointer to a raw steering
> tag value, but you're passing in the steering index to it's table.

Yeah that's weird, there should be one TPH for a DMABUF, not many.

> But in general, since you're letting the user put whatever they want in
> the vfio private area, should there be some validation that it's in the
> valid range? I'm also not quite sure how user space comes to know what
> steering tag to use, or what harm might happen if the wrong one is used.

If the device is VFIO compatible then it needs to ensure that whatever
it does with its steering tags fit the security model of VFIO. You
can't harm the device - you can't reach outside the VFIO sandbox (eg
into another VF or something) and so on.

Under these conditions the kernel doesn't care what TPH is used, just
let userspace specify the raw bits on the wire.

Jason