On Fri, Aug 25, 2023 at 2:30 AM Fangrui Song <maskray@xxxxxxxxxx> wrote: > > On Wed, Aug 23, 2023 at 4:13 PM Denis Nikitin <denik@xxxxxxxxxxxx> wrote: > > > > On Wed, Aug 23, 2023 at 4:02 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > > > > > > On Wed, Aug 23, 2023 at 3:00 AM Fangrui Song <maskray@xxxxxxxxxx> wrote: > > > > > > > > On Tue, Aug 22, 2023 at 10:49 AM Denis Nikitin <denik@xxxxxxxxxxxx> wrote: > > > > > > > > > > .llvm.call-graph-profile section is added by clang when the kernel is > > > > > built with profiles (e.g. -fprofile-sample-use= or -fprofile-use=). > > > > > > > > > > The section contains edge information derived from text sections, > > > > > so .llvm.call-graph-profile itself doesn't need more analysis as > > > > > the text sections have been analyzed. > > > > > > > > > > This change fixes the kernel build with clang and a sample profile > > > > > which currently fails with: > > > > > > > > > > "FATAL: modpost: Please add code to calculate addend for this architecture" > > > > > > > > > Curious. > > > > > > This message is only displayed for REL. > > > > > > (Please not it is located in section_rel() function) > > > > > > > > > I think modern architectures use RELA instead of REL. > > > Which architecture are we talking about? > > > > Aarch64. There was also a report on x86-64 but the error message could be > > different there. > > Regarding REL: > > The original format of .llvm.call-graph-profile > (SHT_LLVM_CALL_GRAPH_PROFILE=0x6fff4c02) used symbol indices without > relocations and could be corrupted by symbol table change. > https://github.com/llvm/llvm-project/commit/a224c5199b327ed0efcdcd87b6dbf950cf4d9ee1 > (2021) changed the format to represent call edge information with > R_*_NONE and changed SHT_LLVM_CALL_GRAPH_PROFILE to 0x6fff4c09. > > We don't use the addend field of R_*_NONE relocations, so I proposed > that we use REL for all targets. > My https://github.com/llvm/llvm-project/commit/ca3bdb57fa1ac98b711a735de048c12b5fdd8086 > selected REL for .llvm.call-graph-profile > > aaelf64 says: > > > A binary file may use ``REL`` or ``RELA`` relocations or a mixture of the two (but multiple relocations of the same place must use only one type). > > Other psABI documents may be more vague on how REL is used, but as > long as the tool that needs to process it (currently just lld and > readelf like tools) supports it, it's fine. > binutils seems to support REL for all ELF targets, even if its > objcopy/strip may unintentionally convert REL to RELA. lld can consume > RELA SHT_LLVM_CALL_GRAPH_PROFILE. > > > > > > > > > > What does the output of this command look like? > > > > > > $ llvm-readelf -S vmlinux.o | grep .llvm.call-graph-profile > > > > > > > > > Is it REL? > > > > > > > [119] .llvm.call-graph-profile LLVM_CALL_GRAPH_PROFILE 0000000000000000 > > 1c74a458 0104c8 08 E 0 0 1 > > [120] .rel.llvm.call-graph-profile REL 0000000000000000 1c75a920 041320 10 > > I 26090 119 8 > > > > Thanks, > > Denis Thanks, Fangrui. I'd like the commit log to record the use of REL for .llvm.call-graph-profile is intentional. Denis, could you add some more comments? -- Best Regards Masahiro Yamada
