Re: [RFC PATCH v3 06/11] drm/pagemap: Add IOVA interface to DRM pagemap

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jan 28, 2026 at 12:24:25PM -0800, Matthew Brost wrote:
> On Wed, Jan 28, 2026 at 03:35:09PM -0400, Jason Gunthorpe wrote:
> > On Wed, Jan 28, 2026 at 10:42:53AM -0800, Matthew Brost wrote:
> > > Yes, this is exactly what I envision here. First, let me explain the
> > > possible addressing modes on the UAL fabric:
> > > 
> > >  - Physical (akin to IOMMU passthrough)
> > >  - Virtual (akin to IOMMU enabled)
> > > 
> > > Physical mode is straightforward — resolve the PFN to a cross-device
> > > physical address, then install it into the initiator’s page tables along
> > > with a bit indicating routing over the network. In this mode, the vfuncs
> > > here are basically NOPs.
> > 
> > Ugh of course they would invent something so complicated.
> 
> Why wouldn't we... But conceptually really fairly close to IOMMU
> paththrough vs. enabled.

Why do you need address virtualization on the scale up fabric :( I can
see access control but full virtualization sounds like overkill,
especially considering how slow it will necessarily be compared to the
fabric itself.

We are already in a world where even PCI can't manage untranslated
requests and a scale up fabric with 3TB/sec of bandwidth is somehow
going to have address translation too? Doesn't seem reasonable.

> > I'm not convinced this should be hidden inside DRM. The DMA API is the
> 
> 
> Well, what I’m suggesting isn’t in DRM. A UAL API would be its own
> layer, much like the DMA API. Of course we could stick this in the DMA
> API and make it high-speed-fabric-generic, etc., but I do think the
> fabric functions would have their own signatures and semantics (see my
> explanation around device_ual_alloc reclaim rules, what locks it is
> allowed to take, etc.).

DMA API is already bus agnostic, I think there is no issue to plug in
a ualink_device or whatever under there and make it do something
sensible, and it would be *particularly* easy if the address
translation can slot in as an attached iommu.

Jason