Re: [dpdk-dev] [PATCH v1 1/6] net/af_xdp: introduce AF_XDP PMD driver

[Ye Xiaolong <xiaolong.ye@intel.com>]

On 03/25, Luca Boccassi wrote:
>On Mon, 2019-03-25 at 10:45 +0800, Ye Xiaolong wrote:
>> On 03/24, Luca Boccassi wrote:
>> > On Sun, 2019-03-17 at 11:34 +0800, Ye Xiaolong wrote:
>> > > On 03/02, Ye Xiaolong wrote:
>> > > > > >  _LDLIBS-$(CONFIG_RTE_LIBRTE_PMD_AF_PACKET)  +=
>> > > > > > -lrte_pmd_af_packet
>> > > > > > +_LDLIBS-$(CONFIG_RTE_LIBRTE_PMD_AF_XDP)     +=
>> > > > > > -lrte_pmd_af_xdp
>> > > > > > -lelf -lbpf
>> > > > > 
>> > > > > Are symbols from libelf being used by the PMD?
>> > > > 
>> > > > Hmm, it is a leftover of RFC, libelf is no longer needed in
>> > > > this
>> > > > version, will
>> > > > remove it in next version.
>> > > > 
>> > > 
>> > > Correction, libelf is needed for libbpf, so we still need to keep
>> > > it. 
>> > 
>> > If libbpf needs libelf for its internal usage, it should be linked
>> > against it itself. Unless symbols from libelf are used in static
>> > functions defined in libbpf's public headers. Is this the case?
>> > 
>> 
>> Yes, that's the case. without the libelf, it would have build error
>> as below,
>> and these symbols are used in static functions of libbpf.
>> 
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_nextscn'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_rawdata'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_memory'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `gelf_getrel'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_strptr'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_end'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_getscn'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_begin'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `gelf_getsym'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_version'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `gelf_getehdr'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `elf_getdata'
>> /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/libbpf.so:
>> undefined reference to `gelf_getshdr'
>> 
>> Thanks,
>> Xiaolong
>
>Looking at that list, at least the very first symbol is not used by a
>public header, but internally in libbpf:
>
>$ grep -r elf_nextscn
>libbpf.c:	while ((scn = elf_nextscn(elf, scn)) != NULL) {
>
>https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/lib/bpf/libbpf.c#n793
>
>None of those symbols are used from the public headers:
>
>$ grep elf_ bpf.h libbpf.h btf.h
>$
>
>Therefore, this is an omission in libbpf's Makefile, as it must link
>against libelf. Please raise a ticket or send a patch upstream, and
>remove the -lelf from DPDK's makefiles.

As it may need sometime for kernel community to handle the libbpf's Makefile, 
We may still need the -lelf for af_xdp pmd currently, I'll remove it later after
libbpf correct to link against libelf. Is it acceptable?

Thanks,
Xiaolong
>
>-- 
>Kind regards,
>Luca Boccassi