Re: [RFC][PATCH 1/2] x86/retpoline: Simplify retpolines

[Peter Zijlstra]

On Thu, Feb 18, 2021 at 05:59:39PM +0100, Peter Zijlstra wrote:
> Currently out retpolines consist of 2 symbols,
> __x86_indirect_thunk_\reg, which is the compiler target, and
> __x86_retpoline_\reg, which is the actual retpoline. Both are
> consecutive in code and aligned such that for any one register they
> both live in the same cacheline:
> 
>   0000000000000000 <__x86_indirect_thunk_rax>:
>    0:   ff e0                   jmpq   *%rax
>    2:   90                      nop
>    3:   90                      nop
>    4:   90                      nop
> 
>   0000000000000005 <__x86_retpoline_rax>:
>    5:   e8 07 00 00 00          callq  11 <__x86_retpoline_rax+0xc>
>    a:   f3 90                   pause
>    c:   0f ae e8                lfence
>    f:   eb f9                   jmp    a <__x86_retpoline_rax+0x5>
>   11:   48 89 04 24             mov    %rax,(%rsp)
>   15:   c3                      retq
>   16:   66 2e 0f 1f 84 00 00 00 00 00   nopw   %cs:0x0(%rax,%rax,1)
> 
> The thunk is an alternative_2, where one option is a jmp to the
> retpoline.

So the reason I originally did that was because objtool could not deal
with alternatives with stack ops. But we've recently fixed that.