On Tue, Aug 22, 2017 at 11:30 AM, Duyck, Alexander H <alexander.h.duyck@xxxxxxxxx> wrote: > On Tue, 2017-08-22 at 11:17 -0700, John Fastabend wrote: >> On 08/22/2017 11:02 AM, Michael Chan wrote: >> > On Mon, Aug 21, 2017 at 12:25 PM, Jesper Dangaard Brouer >> > <brouer@xxxxxxxxxx> wrote: >> > > >> > > I'be been playing with the latest XDP_REDIRECT feature, that was >> > > accepted in net-next (for ixgbe), see merge commit[1]. >> > > [1] https://git.kernel.org/davem/net-next/c/6093ec2dc31 >> > > >> > >> > Just catching on XDP_REDIRECT and I have a very basic question. The >> > ingress device passes the XDP buffer to the egress device for XDP >> > redirect transmission. When the egress device has transmitted the >> > packet, is it supposed to just free the buffer? Or is it supposed to >> > be recycled? >> > >> > In XDP_TX, the buffer is recycled back to the rx ring. >> > >> >> With XDP_REDIRECT we must "just free the buffer" in ixgbe this means >> page_frag_free() on the data. There is no way to know where the xdp >> buffer came from it could be a different NIC for example. >> >> However with how ixgbe is coded up recycling will work as long as >> the memory is free'd before the driver ring tries to use it again. In >> normal usage this should be the case. And if we are over-running a device >> it doesn't really hurt to slow down the sender a bit. >> >> I think this is a pretty good model, we could probably provide a set >> of APIs for drivers to use so that we get some consistency across >> vendors here, ala Jesper's page pool ideas. >> >> (+Alex, for ixgbe details) >> >> Thanks, >> John > > I think you pretty much covered the inner workings for the ixgbe bits. > > The only piece I would add is that the recycling trick normally only > works if the same interface/driver is doing both the Tx and the Rx. The > redirect code cannot assume that is the case and that is the reason why > it must always be freeing the traffic on clean-up. > Right, but it's conceivable to add an API to "return" the buffer to the input device, right?
|