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Date: Sat, 13 Jan 2018 22:36:13 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Tom Lendacky <thomas.lendacky@amd.com>
cc: "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
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Subject: Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling
 RSB macros
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On Sat, 13 Jan 2018, Tom Lendacky wrote:

> On 1/13/2018 8:07 AM, Van De Ven, Arjan wrote:
> >>> The RSB filling macro is applicable to AMD, and, if software is unable to
> >>> verify that lfence is serializing on AMD (possible when running under a
> >>> hypervisor), the generic retpoline support will be used and, so, is also
> >>> applicable to AMD.  Change the use of pause to lfence.
> >>>
> >>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> >>
> >> Conditionally-Acked-by: David Woodhouse <dwmw@amazon.co.uk>
> > 
> > 
> > pause is technically the "save me power" instruction
> > 
> > how about a compromise where we do a double:
> > 
> > pause
> > lfence
> > jmp <up>
> > 
> > as sequence... that way if the branch recovery is fast, we get the performance of pause, but if it takes a while, on AMD you get the behavior of lfence?
> 
> That should work on AMD.

I zapped the commit from tip for now until this discussion is resolved.

Thanks,

	tglx
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