On 27/10/2021 13.35, Toke Høiland-Jørgensen wrote:
Jun Hu <hujun.work@xxxxxxxxx> writes:On Tue, Oct 26, 2021 at 4:08 AM Konrad Zemek <konrad@xxxxxxxx> wrote:On Monday, October 25th, 2021 at 8:26 PM, Jun Hu <hujun.work@xxxxxxxxx> wrote:On Mon, Oct 25, 2021 at 3:48 AM Toke Høiland-Jørgensen toke@xxxxxxxxxx wrote:Jun Hu hujun.work@xxxxxxxxx writes:Hi, I want to put a timestamp in the egress packets, and I want to use AF_XDP socket for both sending and receiving packets; since a XDPkernel program only work in ingress direction, I wonder if I could usea tc kernel program to change a packet sent out via an AF_XDP socket?I searched around, but couldn't find any direct answer...Nope, AF_XDP bypasses the stack completely, so TC won't see the packeton egress... -TokeOk, thanks for the clarification, I guess I have to wait for the XDPegress support ...You can also directly modify the packet that you send out through an AF_XDP socket, before putting it on the outgoing queue. Is there anything specific that you would want to do in an egress-attached BPF program that you would not be able to do in the userspace program that manages your AF_XDP socket? KonradYes, that's what I am doing right now, however ideally I want to add a timestamp to the packet as close as it egress the NIC; since AF_XDP doesn't have access to hardware timestamp, that's why I was searching for an alternative solution.Cc. Jesper, as I believe timestamps is one of his initial use cases for xdp-hints (hardware metadata).
Correct, Toke. I have a use-case/customer that want timestamps working with AF_XDP. And yes the plan is to leverage XDP-hints (via XDP metadata area) to expose/export these different packet timestamps. I have two use-case for timestamps: (1) RX-timestamping. (2) TX-launch-time. This week, I have promised to implement a software version of (1) RX-timestamping, where XDP will add ktime into the metadata area and AF_XDP userspace program will consume this. It is rather trivial to do, but I want extend it and use it to build a flexible BTF-info variant on-top (which doesn't requires any kernel changes). The goal is to get the hardware RX-timestamp, via changing the NIC driver to populate the XDP-hints in metadata areas (from HW-RX-descriptor). Again using BTF to describe XDP-hints contents, then AF_XDP userspace prog can hopefully use the same code to find and extract members related to the HW RX-timestamp. For (2) TX-launch-time, as you mention XDP is missing an egress hook orqueueing layer. Thus, there are some kernel work a-head. Toke and a PhD stud is looking into this area.
As discussed/described before on xdp-hints mailing list[1] (cc'ed), in this step it is the AF_XDP program that updates the XDP-hints areabefore sending it, and the (non-existing XDP egress) driver hook moves requested LaunchTime into the HW-TX-descriptor.
[1] https://lists.xdp-project.net/postorius/lists/xdp-hints.xdp-project.net/
What is your use-case for adding a TX-timestamp to the packet? --Jesper
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