On Thu, Oct 31, 2019 at 1:03 AM Björn Töpel <bjorn.topel@xxxxxxxxx> wrote: > > On Thu, 31 Oct 2019 at 08:17, Magnus Karlsson <magnus.karlsson@xxxxxxxxx> wrote: > > > > On Wed, Oct 30, 2019 at 2:36 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > > > > > Magnus Karlsson <magnus.karlsson@xxxxxxxxx> writes: > > > > > > > When the need_wakeup flag was added to AF_XDP, the format of the > > > > XDP_MMAP_OFFSETS getsockopt was extended. Code was added to the > > > > kernel to take care of compatibility issues arrising from running > > > > applications using any of the two formats. However, libbpf was > > > > not extended to take care of the case when the application/libbpf > > > > uses the new format but the kernel only supports the old > > > > format. This patch adds support in libbpf for parsing the old > > > > format, before the need_wakeup flag was added, and emulating a > > > > set of static need_wakeup flags that will always work for the > > > > application. > > > > > > Hi Magnus > > > > > > While you're looking at backwards compatibility issues with xsk: libbpf > > > currently fails to compile on a system that has old kernel headers > > > installed (this is with kernel-headers 5.3): > > > > > > $ echo "#include <bpf/xsk.h>" | gcc -x c - > > > In file included from <stdin>:1: > > > /usr/include/bpf/xsk.h: In function ‘xsk_ring_prod__needs_wakeup’: > > > /usr/include/bpf/xsk.h:82:21: error: ‘XDP_RING_NEED_WAKEUP’ undeclared (first use in this function) > > > 82 | return *r->flags & XDP_RING_NEED_WAKEUP; > > > | ^~~~~~~~~~~~~~~~~~~~ > > > /usr/include/bpf/xsk.h:82:21: note: each undeclared identifier is reported only once for each function it appears in > > > /usr/include/bpf/xsk.h: In function ‘xsk_umem__extract_addr’: > > > /usr/include/bpf/xsk.h:173:16: error: ‘XSK_UNALIGNED_BUF_ADDR_MASK’ undeclared (first use in this function) > > > 173 | return addr & XSK_UNALIGNED_BUF_ADDR_MASK; > > > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > /usr/include/bpf/xsk.h: In function ‘xsk_umem__extract_offset’: > > > /usr/include/bpf/xsk.h:178:17: error: ‘XSK_UNALIGNED_BUF_OFFSET_SHIFT’ undeclared (first use in this function) > > > 178 | return addr >> XSK_UNALIGNED_BUF_OFFSET_SHIFT; > > > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > > > > > > > > > How would you prefer to handle this? A patch like the one below will fix > > > the compile errors, but I'm not sure it makes sense semantically? > > > > Thanks Toke for finding this. Of course it should be possible to > > compile this on an older kernel, but without getting any of the newer > > functionality that is not present in that older kernel. > > Is the plan to support source compatibility for the headers only, or > the whole the libbpf itself? Is the usecase here, that you've built > libbpf.so with system headers X, and then would like to use the > library on a system with older system headers X~10? XDP sockets? BTF? libbpf has to be backward and forward compatible. Once compiled it has to run on older and newer kernels. Conditional compilation is not an option obviously.
