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

[David Woodhouse]

On Thu, 2018-01-04 at 10:36 -0800, Alexei Starovoitov wrote:
> 
> Pretty much.
> Paul's writeup: https://support.google.com/faqs/answer/7625886
> tldr: jmp *%r11 gets converted to:
> call set_up_target;
> capture_spec:
> Â pause;
> Â jmp capture_spec;
> set_up_target:
> Â mov %r11, (%rsp);
> Â ret;
> where capture_spec part will be looping speculatively.

That is almost identical to what's in my latest patch set, except that
the capture_spec loop has 'lfence' instead of 'pause'.

As Andi says, I'd want to see explicit approval from the CPU architects
for making that change.

We've already had false starts there â for a long time, Intel thought
that a much simpler option with an lfence after the register load was
sufficient, and then eventually worked out that in some rare cases it
wasn't. While AMD still seem to think it *is* sufficient for them,
apparently.

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