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

[Miroslav Benes]

On Tue, 26 Oct 2021, Peter Zijlstra 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

It is already in tip, but FWIW the objtool changes look good to me

Reviewed-by: Miroslav Benes <mbenes@xxxxxxx>

M