On 17-01-19 07:26 PM, Jason Wang wrote: > > > On 2017年01月20日 05:11, Michael S. Tsirkin wrote: >> 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. >> > > Maybe I was wrong but I think driver should try their best to avoid dropping > packets. (And look at mlx4, it did something similar to this patch). > > Thanks
+1 sorry didn't see your reply as I was typing mine. Bottom line when XDP returns I believe the driver must be ready to accept packets or managing XDP will be problematic. .John
