Re: [RFC PATCH 00/10] Device Memory TCP

[Andy Lutomirski]

On 7/10/23 15:32, Mina Almasry wrote:
> * TL;DR:
>
> Device memory TCP (devmem TCP) is a proposal for transferring data to and/or
> from device memory efficiently, without bouncing the data to a host memory
> buffer.

(I'm writing this as someone who might plausibly use this mechanism, but 
I don't think I'm very likely to end up working on the kernel side, 
unless I somehow feel extremely inspired to implement it for i40e.)

I looked at these patches and the GVE tree, and I'm trying to wrap my 
head around the data path.  As I understand it, for RX:

1. The GVE driver notices that the queue is programmed to use devmem, 
and it programs the NIC to copy packet payloads to the devmem that has 
been programmed.
2. The NIC receives the packet and copies the header to kernel memory 
and the payload to dma-buf memory.
3. The kernel tells userspace where in the dma-buf the data is.
4. Userspace does something with the data.
5. Userspace does DONTNEED to recycle the memory and make it available 
for new received packets.

Did I get this right?

This seems a bit awkward if there's any chance that packets not intended 
for the target device end up in the rxq.

I'm wondering if a more capable if somewhat higher latency model could 
work where the NIC stores received packets in its own device memory. 
Then userspace (or the kernel or a driver or whatever) could initiate a 
separate DMA from the NIC to the final target *after* reading the 
headers.  Can the hardware support this?

Another way of putting this is: steering received data to a specific 
device based on the *receive queue* forces the logic selecting a 
destination device to be the same as the logic selecting the queue.  RX 
steering logic is pretty limited on most hardware (as far as I know -- 
certainly I've never had much luck doing anything especially intelligent 
with RX flow steering, and I've tried on a couple of different brands of 
supposedly fancy NICs).  But Linux has very nice capabilities to direct 
packets, in software, to where they are supposed to go, and it would be 
nice if all that logic could just work, scalably, with device memory. 
If Linux could examine headers *before* the payload gets DMAed to 
wherever it goes, I think this could plausibly work quite nicely.  One 
could even have an easy-to-use interface in which one directs a *socket* 
to a PCIe device.  I expect, although I've never looked at the 
datasheets, that the kernel could even efficiently make rx decisions 
based on data in device memory on upcoming CXL NICs where device memory 
could participate in the host cache hierarchy.

My real ulterior motive is that I think it would be great to use an 
ability like this for DPDK-like uses.  Wouldn't it be nifty if I could 
open a normal TCP socket, then, after it's open, ask the kernel to 
kindly DMA the results directly to my application memory (via udmabuf, 
perhaps)?  Or have a whole VLAN or macvlan get directed to a userspace 
queue, etc?


It also seems a bit odd to me that the binding from rxq to dma-buf is 
established by programming the dma-buf.  This makes the security model 
(and the mental model) awkward -- this binding is a setting on the 
*queue*, not the dma-buf, and in a containerized or privilege-separated 
system, a process could have enough privilege to make a dma-buf 
somewhere but not have any privileges on the NIC.  (And may not even 
have the NIC present in its network namespace!)

--Andy