Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

[Brian Gerst]

On Fri, Jan 5, 2018 at 3:32 PM, Woodhouse, David <dwmw@xxxxxxxxxxxx> wrote:
> On Fri, 2018-01-05 at 09:28 -0800, Linus Torvalds wrote:
>>
>> Yes, I would suggest against expecting altinstructions to have
>> relocation information. They are generated in a different place, so..
>>
>> That said, I honestly like the inline version (the one that is in the
>> google paper first) of the retpoline more than the out-of-line one.
>> And that one shouldn't have any relocation issues, because all the
>> offsets are relative.
>
> Note that the *only* issue with the relocation is that it pushes me to
> use X86_FEATURE_NO_RETPOLINE for my feature instead of
> X86_FEATURE_RETPOLINE as might be natural. And actually there's a
> motivation to do that anyway, because of the way three-way alternatives
> interact.
>
> With the existing negative flag I can do
>
>  ALTERNATIVE_2(retpoline, K8: lfence+jmp; NO_RETPOLINE: jmp)
>
> But if I invert it, I think I need two feature flags to get the same functionality â X86_FEATURE_RETPOLINE and X86_FEATURE_RETPOLINE_AMD:
>
>  ALTERNATIVE_2(jmp, RETPOLINE: retpoline, RETPOLINE_AMD: lfence+jmp)

Another way to do it is with two consecutive alternatives:

ALTERNATIVE(NOP, K8: lfence)
ALTERNATIVE(jmp indirect, RETPOLINE: jmp thunk)

This also avoids the issue with the relocation of the jmp target when
the replacement is more than one instruction.

--
Brian Gerst