On Tue, Aug 31, 2021 at 4:10 PM Indu Bhagat via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > [Changes from V2] > - Instead of target hook, the patch set now adds a new debug format > BTF_WITH_CORE_DEBUG. > - Renamed the BPF option from -mcore to -mco-re. > - Adapted the commit logs a bit. > [End of Changes from V2]
This version of the series is OK for trunk. Thanks, Richard. > > Hello, > > This patch series puts the framework in place for late BTF generation (in > dwarf2out_finish). This is needed for the landing of BPF CO-RE support in GCC, > patches for which were posted earlier - > https://gcc.gnu.org/pipermail/gcc-patches/2021-August/576719.html. > > BPF's Compile Once - Run Everywhere (CO-RE) feature is used to make a compiled > BPF program portable across kernel versions, all this without the need to > recompile the BPF program. A key part of BPF CO-RE capability is the BTF debug > info generated for them. > > A traditional BPF program (non CO-RE) will have a .BTF section which contains > the type information in the BTF debug format. In case of CO-RE, however, an > additional section .BTF.ext section is used. The .BTF.ext section contains > the CO-RE relocations. A BPF loader will use the .BTF.ext section along with > the > associated .BTF section to adjust some references in the instructions of BPF > program to ensure it is compatible with the required kernel version / headers. > > A .BTF.ext section contains the CO-RE relocation records. Roughly, each CO-RE > relocation record will contain the following info: > - offset of BPF instruction to be patched > - the BTF ID of the data structure being accessed by the instruction, and > - an offset to the "access string" - a BTF string which encodes a series of > field accesses to retrieve the field of interest in the instruction. > > The .BTF.ext section does not have a string table of its own, so these "access > strings" are placed in the .BTF section string table. The CO-RE relocation > records refer to them by offset into the .BTF string table. > > Example of access string encoding: > > struct S { > int a; > union { > int _unused; > int b; > char c; > } u[4]; > }; > > struct S *foo = ...; > int x = foo->a; /* encoded as "0:0" */ > int y = foo[4]->u[2].b /* encoded as "4:1:2:1" */ > char z = foo->u[3].c /* encoded as "0:1:3:2" */ > > This means that writing out of a .BTF section needs to be delayed until after > these "access strings" have been added by the BPF backend, when CO-RE is in > effect. > > High-level design > ----------------- > - The CTF container (CTFC) is populated with the compiler-internal > representation for the "type information" at dwarf2out_early_finish time. > This > information is used for generation of the .BTF section. > - For CO-RE relocation records, the information needed to generate .BTF.ext > section is added by the BPF backend to the CTF container (CTFC) at expand > time. > - A new debug format BTF_WITH_CORE_DEBUG is being added. > - The BPF backend updates the write_symbols variable with BTF_WITH_CORE_DEBUG > debug format signalling the rest of the compiler that BPF/CO-RE is in effect, > and hence the need to generate the BTF CO-RE relocation records. > - BTF debug information is emitted late in dwarf2out_finish when > BTF_WITH_CORE_DEBUG debug format is requested by the user (implicitly via the > command line option -mco-re for the BPF backend). > - Combining early BTF and late BTF/CO-RE debug information is not feasible due > to the way BTF CO-RE format is defined and lack of linker support for the BTF > debug format. > - Lastly, LTO is disallowed to be used together with CO-RE for the BPF target. > > Testing Notes > ------------ > - Bootstrapped and reg tested on x86_64 > - make all-gcc for --target=bpf-unknown-none; tested ctf.exp, btf.exp and > bpf.exp > > Thanks, > > Indu Bhagat (3): > debug: add BTF_WITH_CORE_DEBUG debug format > bpf: Add new -mco-re option for BPF CO-RE > dwarf2out: Emit BTF in dwarf2out_finish for BPF CO-RE usecase > > gcc/config/bpf/bpf.c | 25 ++++++++++++++ > gcc/config/bpf/bpf.opt | 4 +++ > gcc/dwarf2ctf.c | 54 > +++++++++++++++++++++++-------- > gcc/dwarf2ctf.h | 4 ++- > gcc/dwarf2out.c | 9 ++++-- > gcc/flag-types.h | 6 +++- > gcc/flags.h | 4 +++ > gcc/opts.c | 8 +++++ > gcc/testsuite/gcc.target/bpf/core-lto-1.c | 9 ++++++ > 9 files changed, 106 insertions(+), 17 deletions(-) > create mode 100644 gcc/testsuite/gcc.target/bpf/core-lto-1.c > > -- > 1.8.3.1 >
