Re: [PATCH v3 00/16] x86: Rewrite the retpoline rewrite logic

[Alexei Starovoitov]

On Tue, Oct 26, 2021 at 5:05 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Hi,
>
> These patches rewrite the way retpolines are rewritten. Currently objtool emits
> alternative entries for most retpoline calls. However trying to extend that led
> to trouble (ELF files are horrid).
>
> Therefore completely overhaul this and have objtool emit a .retpoline_sites
> section that lists all compiler generated retpoline thunk calls. Then the
> kernel can do with them as it pleases.
>
> Notably it will:
>
>  - rewrite them to indirect instructions for !RETPOLINE
>  - rewrite them to lfence; indirect; for RETPOLINE_AMD,
>    where size allows (boo clang!)
>
> Specifically, the !RETPOLINE case can now also deal with the clang-special
> conditional-indirect-tail-call:
>
>   Jcc __x86_indirect_thunk_\reg.
>
> Finally, also update the x86 BPF jit to catch up to recent times and do these
> same things.
>
> All this should help improve performance by removing an indirection.
>
> Patches can (soon) be found here:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git objtool/core
>
> Changes since v2:
>
>  - rewrite the __x86_indirect_thunk_array[] stuff again
>  - rewrite the retpoline,amd rewrite logic, it now also supports
>    rewriting the Jcc case, if the original instruction is long enough, but
>    more importantly, it's simpler code.
>  - bpf label simplification patch
>  - random assorted cleanups
>  - actually managed to get bpf selftests working

Great.
The patchset didn't go through BPF CI though.
See
https://patchwork.kernel.org/project/netdevbpf/patch/20211026120309.658539311@xxxxxxxxxxxxx/

It's a merge conflict. The patchset failed to apply to both bpf and
bpf-next trees:

Cmd('git') failed due to: exit code(128)
  cmdline: git am -3
  stdout: 'Applying: objtool: Classify symbols
Patch failed at 0001 objtool: Classify symbols
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".'
  stderr: 'error: sha1 information is lacking or useless
(tools/objtool/check.c).
error: could not build fake ancestor
hint: Use 'git am --show-current-patch=diff' to see the failed patch'