Re: [PATCH v4 0/3] bootconfig: Support embedding a bootconfig in kernel for non initrd boot

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Padmanabha,

On Sun, 27 Mar 2022 08:53:06 +0900
Masami Hiramatsu <mhiramat@kernel.org> 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@kernel.org>


-- 
Masami Hiramatsu <mhiramat@kernel.org>