On Tue, Nov 29, 2016 at 06:50:18PM -0800, John Fastabend wrote: > On 16-11-29 04:37 PM, Alexei Starovoitov wrote: > > On Tue, Nov 29, 2016 at 12:11:33PM -0800, John Fastabend wrote: > >> virtio_net XDP support expects receive buffers to be contiguous. > >> If this is not the case we enable a slowpath to allow connectivity > >> to continue but at a significan performance overhead associated with > >> linearizing data. To make it painfully aware to users that XDP is > >> running in a degraded mode we throw an xdp buffer error. > >> > >> To linearize packets we allocate a page and copy the segments of > >> the data, including the header, into it. After this the page can be > >> handled by XDP code flow as normal. > >> > >> Then depending on the return code the page is either freed or sent > >> to the XDP xmit path. There is no attempt to optimize this path. > >> > >> Signed-off-by: John Fastabend <john.r.fastab...@intel.com> > > ... > >> +/* The conditions to enable XDP should preclude the underlying device from > >> + * sending packets across multiple buffers (num_buf > 1). However per spec > >> + * it does not appear to be illegal to do so but rather just against > >> convention. > >> + * So in order to avoid making a system unresponsive the packets are > >> pushed > >> + * into a page and the XDP program is run. This will be extremely slow > >> and we > >> + * push a warning to the user to fix this as soon as possible. Fixing > >> this may > >> + * require resolving the underlying hardware to determine why multiple > >> buffers > >> + * are being received or simply loading the XDP program in the ingress > >> stack > >> + * after the skb is built because there is no advantage to running it here > >> + * anymore. > >> + */ > > ... > >> if (num_buf > 1) { > >> bpf_warn_invalid_xdp_buffer(); > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > Here is the warn once call. I made it a defined bpf warning so that we > can easily identify if needed and it can be used by other drivers as > well.
ahh. I thought it has - in front of it :) and you're deleting it in this patch. > >> - goto err_xdp; > >> + > >> + /* linearize data for XDP */ > >> + xdp_page = xdp_linearize_page(rq, num_buf, > >> + page, offset, &len); > >> + if (!xdp_page) > >> + goto err_xdp; > > > > in case when we're 'lucky' the performance will silently be bad. > > Were you specifically worried about the alloc failing here and no > indication? I was thinking a statistic counter could be added as a > follow on series to catch this and other such cases in non XDP paths > if needed. > > > Can we do warn_once here? so at least something in dmesg points out > > that performance is not as expected. Am I reading it correctly that > > you had to do a special kernel hack to trigger this situation and > > in all normal cases it's not the case? > > > > Correct the only way to produce this with upstream qemu and Linux is to > write a kernel hack to build these types of buffers. AFAIK I caught all > the cases where it could happen otherwise in the setup xdp prog call and > required user to configure device driver so they can not happen e.g. > disable LRO, set MTU < PAGE_SIZE, etc. > > If I missed anything, I don't see it now, I would call it a bug and fix > it. Again AFAIK everything should be covered though. As Micheal pointed > out earlier although unexpected its not strictly against virtio spec to > build such packets so we should handle it at least in a degraded mode > even though it is not expected in any qemu cases. Ok. Makes sense. The above wasn't clear in the commit log. Thanks