Re: [PATCH rdma-next 0/6] Add support for TLP emulation

[Jason Gunthorpe]

On Fri, Feb 27, 2026 at 02:37:05PM -0700, Keith Busch wrote:
> On Wed, Feb 25, 2026 at 04:19:30PM +0200, Leon Romanovsky wrote:
> > This series adds support for Transaction Layer Packet (TLP) emulation
> > response gateway regions, enabling userspace device emulation software
> > to write TLP responses directly to lower layers without kernel driver
> > involvement.
> > 
> > Currently, the mlx5 driver exposes VirtIO emulation access regions via
> > the MLX5_IB_METHOD_VAR_OBJ_ALLOC ioctl. This series extends that
> > ioctl to also support allocating TLP response gateway channels for
> > PCI device emulation use cases.
> 
> Sorry if this is obvious to people in the know, but could you possibly
> give a quick high level description of the use case behind this feature?
> I'm just curious what emulation needs are enabled by having access to
> this packet level. Thanks!

These days the DPU world supports what I think of as "software defined
PCI functions". Meaning when the DPU receives a PCIe TLP on its PCI
interface it may invoke software generate a response packet for that
TLP.

At least the Mellanox DPU can route the TLPs to software in many
different places: various on-device processors, or on the ARM cores
running Linux..

So, for example, using this basic capability you can write some
software to have the DPU create a PCI function that conforms to the
virtio-net specification. Or NVMe. Or whatever else you dream up.

The peculiar thing is that this is all tightly coupled to RDMA. Eg if
you want your TLP to trigger a DMA from the PCI function then RDMA QPs
and MRs have to be used to execute the DMA.

Jason