David Ahern [off-list ref] writes:

On 2/27/20 4:58 AM, Toke Høiland-Jørgensen wrote:

quoted

 also, an egress program may want to actually know which
ingress iface the packet was first received on. So why not just keep
both fields? Since ifindex 0 is invalid anyway, the field could just be
0 when it isn't known (e.g., egress ifindex on RX, or ingress ifindex if
it comes from the stack)?

Today, the ingress device is lost in the conversion from xdp_buff to
xdp_frame. The plumbing needed to keep that information is beyond the
scope of this set.

I am open to making the UAPI separate entries if there is a real reason
for it. Do you have a specific use case?

I was thinking it could be a nice shorthand for whether a packet comes
from the local stack or was forwarded (0 == stack). But no, I don't have
a concrete application where this is useful. However, if we define it as
a union we lose the ability to change our mind. Together with the
debugability issue I just replied with to your other email, I think it
would be better to expend the four bytes keep them as separate fields,
but still restrict access to the RX ifindex for now.

-Toke