Re: AF_XDP socket TX path interact with TC?

Jun Hu <[email protected]> writes:

> On Tue, Oct 26, 2021 at 4:08 AM Konrad Zemek <[email protected]> wrote:
>>
>> On Monday, October 25th, 2021 at 8:26 PM, Jun Hu <[email protected]> wrote:
>>
>> > On Mon, Oct 25, 2021 at 3:48 AM Toke Høiland-Jørgensen [email protected] wrote:
>> >
>> > > Jun Hu [email protected] 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 XDP
>> > > >
>> > > > kernel program only work in ingress direction, I wonder if I could use
>> > > >
>> > > > a 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 packet
>> > >
>> > > on egress...
>> > >
>> > > -Toke
>> >
>> > Ok, thanks for the clarification, I guess I have to wait for the XDP
>> >
>> > egress 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?
>>
>> Konrad
>
> Yes, 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).

-Toke