Hi Padmanabha, On Sun, 27 Mar 2022 08:53:06 +0900 Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > KNOWN ISSUE: > > According to the report from Padmanabha[3], the embedded bootconfig data may not > be updated if you do incremental build the kernel with CONFIG_LTO. This is > under investigation. I tried to test this version with LTO_CLANG_FULL and LTO_CLANG_THIN with switching the embedded bootconfig file by CONFIG_EMBED_BOOT_CONFIG_FILE (on x86). I found that when I use LTO_CLANG_FULL, the embedded bootconfig was updated correctly. But with the LTO_CLANG_THIN, the embedded bootconfig was *NOT* updated. I used the latest prebuild llvm 15.0.0 on x86 [4]. Padmanabha, can you confirm with this latest LLVM? I guess something wrong with your old LLVM. [4] https://download.01.org/0day-ci/cross-package/clang-latest/clang-latest/clang.tar.xz Here is the test procedure. 1. Prepare 2 different bootconfig files (bconf1, bconf2). 2. Configure kernel with LTO_CLANG and setting the full path of bconf1 to CONFIG_EMBED_BOOT_CONFIG_FILE. 3. Build the kernel 4. Boot the kernel with "bootconfig" in the kernel cmdline. 5. Check the /proc/bootconfig is same as bconf1. 6. Reconfigure kernel with the full path of *bconf2* to CONFIG_EMBED_BOOT_CONFIG_FILE. 7. Rebuild the kernel (no cleanup) 8. Boot the kernel with "bootconfig" in the kernel cmdline. 9. Check the /proc/bootconfig is same as bconf2. So with LTO_CLANG_FULL, at the step 9 /proc/bootconfig shows bconf2, but with LTO_CLANG_THIN, it shows bconf1. In both cases, build log showed that the default.bconf was updated (I confirmed the lib/default.bconf is updated) UPD lib/default.bconf CC lib/bootconfig.o AR lib/lib.a Here is my guess. I found that when we enable LTO_CLANG, the compiler compiles C source file into LLVM IR bitcode. $ file work/linux/build-x86_64/lib/bootconfig.o work/linux/build-x86_64/lib/bootconfig.o: LLVM IR bitcode This means at this point the object file doesn't include the lib/default.bconf because it will be embedded by assembler. The bitcode seems only have the inline asm code (which only has an .incbin directive) as a constatns block[5]. [5] Block ID #11 (CONSTANTS_BLOCK): Num Instances: 32 Total Size: 54305b/6788.12B/1697W Percent of file: 19.9792% Average Size: 1697.03/212.13B/53W Tot/Avg SubBlocks: 0/0.000000e+00 Tot/Avg Abbrevs: 4/1.250000e-01 Tot/Avg Records: 486/1.518750e+01 Percent Abbrevs: 80.8642% Record Histogram: Count # Bits b/Rec % Abv Record Kind 219 4860 22.2 100.00 INTEGER 144 1728 12.0 100.00 SETTYPE 41 656 16.0 NULL 39 2970 76.2 CE_INBOUNDS_GEP 26 3504 134.8 100.00 CSTRING 10 37720 3772.0 INLINEASM 4 96 24.0 100.00 CE_CAST 1 58 CE_CMP 1 52 CE_SELECT 1 46 CE_BINOP And when the LLVM runs LTO with THIN mode, it might not update (not rebuild to machine code) that inline asm code block because that block is not updated. I confirmed that the block (bootconfig.o) is not updated after rebuilding the kernel as below. After step 3. $ llvm-bcanalyzer work/linux/build-x86_64/lib/bootconfig.o > bconf.dump1 After step 7. $ llvm-bcanalyzer work/linux/build-x86_64/lib/bootconfig.o > bconf.dump2 $ diff bconf.dump* (No difference) Thank you, > > [3] https://lore.kernel.org/all/20220321183500.GA4065@pswork/T/#u > > Thank you, > > --- > > Masami Hiramatsu (3): > bootconfig: Check the checksum before removing the bootconfig from initrd > bootconfig: Support embedding a bootconfig file in kernel > docs: bootconfig: Add how to embed the bootconfig into kernel > > > Documentation/admin-guide/bootconfig.rst | 30 ++++++++++++++++++++++++++--- > include/linux/bootconfig.h | 10 ++++++++++ > init/Kconfig | 21 ++++++++++++++++++++ > init/main.c | 31 +++++++++++++++--------------- > lib/.gitignore | 1 + > lib/Makefile | 10 ++++++++++ > lib/bootconfig.c | 23 ++++++++++++++++++++++ > 7 files changed, 108 insertions(+), 18 deletions(-) > > -- > Masami Hiramatsu (Linaro) <mhiramat@xxxxxxxxxx> -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
