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

[Thomas Gleixner]

On Tue, 9 Jan 2018, Linus Torvalds wrote:

> 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?

Duh, yes.