Re: [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip

[Linus Torvalds]

On Tue, Jan 9, 2018 at 4:31 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>
> The following patch fixes it for me. Something doesn't
> seem to work with ALTERNATIVE_2. It adds only a few bytes
> more code, so seems acceptable.

Ugh. It's kind of stupid, though.

Why is the code sequence not simply:

  ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE_AMD
  ALTERNATIVE __stringify(jmp *\reg), __stringify(RETPOLINE_JMP \reg),
X86_FEATURE_RETPOLINE

ie make that X86_FEATURE_RETPOLINE_AMD _only_ emit the "lfence", and
simply fall through to what will be the "jmp *\reg" of the
non-RETPOLINE version.

Then just make sure X86_FEATURE_RETPOLINE_AMD disables X86_FEATURE_RETPOLINE.

That is both simpler an dsmaller, no?

                    Linus