Hi Song,

I love your patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/0day-ci/linux/commits/Song-Liu/bpf-introduce-bpf_get_branch_snapshot/20210827-061705
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: arm64-defconfig (attached as .config)
compiler: aarch64-linux-gcc (GCC) 11.2.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/ad355675cf08d95b3f5171b681420cfc8e2f1b7e
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Song-Liu/bpf-introduce-bpf_get_branch_snapshot/20210827-061705
        git checkout ad355675cf08d95b3f5171b681420cfc8e2f1b7e
        # save the attached .config to linux build tree
        mkdir build_dir
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross O=build_dir ARCH=arm64 SHELL=/bin/bash

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <redacted>

All errors (new ones prefixed by >>):

   aarch64-linux-ld: kernel/bpf/trampoline.o: in function `__bpf_prog_enter':

quoted

trampoline.c:(.text+0x61c): undefined reference to `bpf_perf_branch_snapshot'

   aarch64-linux-ld: kernel/bpf/trampoline.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `bpf_perf_branch_snapshot' which may bind externally can not be used when making a shared object; recompile with -fPIC
   trampoline.c:(.text+0x61c): dangerous relocation: unsupported relocation

quoted

aarch64-linux-ld: trampoline.c:(.text+0x624): undefined reference to `bpf_perf_branch_snapshot'

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0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org