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? -Toke
