Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling RSB macros

[Tom Lendacky]

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.ÂÂChange the use of pause to lfence.
>>>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>
>> Conditionally-Acked-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> 
> 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.

Thanks,
Tom

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