Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes: > 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. In this case, it's a constant introduced in the kernel in the current (5.4) cycle; so currently, you can't compile libbpf with kernel-headers-5.3. And we're discussing how to handle this in a backwards compatible way in libbpf... -Toke
