On Thu, Jan 19, 2017 at 11:05:40AM +0800, Jason Wang wrote: > > > On 2017年01月18日 23:15, Michael S. Tsirkin wrote: > > On Tue, Jan 17, 2017 at 02:22:59PM -0800, John Fastabend wrote: > > > Add support for XDP adjust head by allocating a 256B header region > > > that XDP programs can grow into. This is only enabled when a XDP > > > program is loaded. > > > > > > In order to ensure that we do not have to unwind queue headroom push > > > queue setup below bpf_prog_add. It reads better to do a prog ref > > > unwind vs another queue setup call. > > > > > > At the moment this code must do a full reset to ensure old buffers > > > without headroom on program add or with headroom on program removal > > > are not used incorrectly in the datapath. Ideally we would only > > > have to disable/enable the RX queues being updated but there is no > > > API to do this at the moment in virtio so use the big hammer. In > > > practice it is likely not that big of a problem as this will only > > > happen when XDP is enabled/disabled changing programs does not > > > require the reset. There is some risk that the driver may either > > > have an allocation failure or for some reason fail to correctly > > > negotiate with the underlying backend in this case the driver will > > > be left uninitialized. I have not seen this ever happen on my test > > > systems and for what its worth this same failure case can occur > > > from probe and other contexts in virtio framework. > > > > > > Signed-off-by: John Fastabend<john.r.fastab...@intel.com> > > I've been thinking about it - can't we drop > > old buffers without the head room which were posted before > > xdp attached? > > > > Avoiding the reset would be much nicer. > > > > Thoughts? > > > > As been discussed before, device may use them in the same time so it's not > safe. Or do you mean detect them after xdp were set and drop the buffer > without head room, this looks sub-optimal. > > Thanks
Yes, this is what I mean. Why is this suboptimal? It's a single branch in code. Yes we might lose some packets but the big hammer of device reset will likely lose more. -- MST
