On Thu, Oct 31, 2019 at 7:26 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes: > > > On Thu, Oct 31, 2019 at 7:13 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> > >> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes: > >> > >> > 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. > >> > >> So what do we do, then? Redefine the constants in libbpf/xsh.h if > >> they're not in the kernel header file? > > > > why? How and whom it will help? > > To libbpf.rpm creating person or to end user? > > Anyone who tries to compile a new libbpf against an older kernel. You're > saying yourself that "libbpf has to be backward and forward compatible". > Surely that extends to compile time as well as runtime? how old that older kernel? Does it have up-to-date bpf.h in /usr/include ? Also consider that running kernel is often not the same thing as installed in /usr/include vmlinux and /usr/include are different packages.
