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Date: Sun, 14 Jan 2018 16:28:47 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Borislav Petkov <bp@alien8.de>
cc: dwmw@amazon.co.uk, tim.c.chen@linux.intel.com, pjt@google.com,
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Subject: Re: [tip:x86/pti] x86/retpoline: Fill return stack buffer on
 vmexit
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On Sun, 14 Jan 2018, Borislav Petkov wrote:

> On Fri, Jan 12, 2018 at 03:37:49AM -0800, tip-bot for David Woodhouse wrote:
> > Commit-ID:  117cc7a908c83697b0b737d15ae1eb5943afe35b
> > Gitweb:     https://git.kernel.org/tip/117cc7a908c83697b0b737d15ae1eb5943afe35b
> > Author:     David Woodhouse <dwmw@amazon.co.uk>
> > AuthorDate: Fri, 12 Jan 2018 11:11:27 +0000
> > Committer:  Thomas Gleixner <tglx@linutronix.de>
> > CommitDate: Fri, 12 Jan 2018 12:33:37 +0100
> > 
> > x86/retpoline: Fill return stack buffer on vmexit
> 
> ...
> 
> > + /*
> > +  * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
> > +  * monstrosity above, manually.
> > +  */
> > +.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
> > +#ifdef CONFIG_RETPOLINE
> > +	ANNOTATE_NOSPEC_ALTERNATIVE
> > +	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
> > +		__stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))	\
> > +		\ftr
> > +.Lskip_rsb_\@:
> > +#endif
> > +.endm
> 
> Looks unused to me. Was it ever gound to be used? If not:

It's for the outstanding RSB fill after context switch.

Thanks,

	tglx