Re: [PATCH bpf-next v3] libbpf: fix compatibility for kernels without need_wakeup

Alexei Starovoitov
2019-11-01T15:51:56Z

On Fri, Nov 1, 2019 at 12:27 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Thu, Oct 31, 2019 at 01:39:12PM -0700, Alexei Starovoitov wrote:
> > On Thu, Oct 31, 2019 at 12:18 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > > >
> > > > yes. older vmlinux and newer installed libbpf.so
> > > > or any version of libbpf.a that is statically linked into apps
> > > > is something that libbpf code has to support.
> > > > The server can be rebooted into older than libbpf kernel and
> > > > into newer than libbpf kernel. libbpf has to recognize all these
> > > > combinations and work appropriately.
> > > > That's what backward and forward compatibility is.
> > > > That's what makes libbpf so difficult to test, develop and code review.
> > > > What that particular server has in /usr/include is irrelevant.
> > >
> > > sure, anyway we can't compile following:
> > >
> > > tredaell@aldebaran ~ $ 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;
> > > ...
> > >
> > > XDP_RING_NEED_WAKEUP is defined in kernel v5.4-rc1 (77cd0d7b3f257fd0e3096b4fdcff1a7d38e99e10).
> > > XSK_UNALIGNED_BUF_ADDR_MASK and XSK_UNALIGNED_BUF_OFFSET_SHIFT are defined in kernel v5.4-rc1 (c05cd3645814724bdeb32a2b4d953b12bdea5f8c).
> > >
> > > with:
> > > kernel-headers-5.3.6-300.fc31.x86_64
> > > libbpf-0.0.5-1.fc31.x86_64
> > >
> > > if you're saying this is not supported, I guess we could be postponing
> > > libbpf rpm releases until we have the related fedora kernel released
> >
> > why? github/libbpf is the source of truth for building packages
> > and afaik it builds fine.
>
> because we will get issues like above if there's no kernel
> avilable that we could compile libbpf against

what is the issue again?
bpf-next builds fine. github/libbpf builds fine.
If distro is doing something else it's distro's mistake.