Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[David Woodhouse]

On Sun, 2018-02-04 at 19:43 +0100, Thomas Gleixner wrote:
> 
> __x86_return_thunk would look like this:
> 
> __x86_return_thunk:
> ÂÂÂÂÂÂÂÂtestlÂÂÂ$0xf, PER_CPU_VAR(call_depth)
> ÂÂÂÂÂÂÂÂjnzÂÂÂÂÂ1fÂÂÂÂÂÂ
> ÂÂÂÂÂÂÂÂstuff_rsb
> ÂÂ 1:
> ÂÂÂÂÂÂÂÂdeclÂÂÂÂPER_CPU_VAR(call_depth)
> ÂÂÂÂÂÂÂÂret
> 
> The call_depth variable would be reset on context switch.

Note that the 'jnz' can be predicted taken there, allowing the CPU to
speculate all the way to the 'ret'... and beyond.

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