On 05/18/2018 07:29 AM, speck for Thomas Gleixner wrote:
> On Fri, 18 May 2018, speck for Konrad Rzeszutek Wilk wrote:
>> On Thu, May 17, 2018 at 10:53:28PM +0200, speck for Thomas Gleixner wrote:
>>> Folks,
>>>
>>> we finally reached a stable state with the SSB patches. I've updated all 3
>>> branches master/linux-4.16.y/linux-4.14.y in the repo and attached the
>>> resulting git bundles. They merge cleanly on top of the current HEADs of
>>> the relevant trees.
>>>
>>> The lot survived light testing on my side and it would be great if everyone
>>> involved could expose it to their test scenarios.
>>>
>>> Thanks to everyone who participated in that effort (patches, review,
>>> testing ...)!
>>
>> Yeey! Thank you.
>>
>> I was reading the updated Intel doc today (instead of skim reading it) and it mentioned:
>>
>> "Intel recommends that the SSBD MSR bit be cleared when in a sleep state on such processors."
> 
> Well, the same recommendation was for IBRS and the reason is that with HT
> enabled the other hyperthread will not be able to go full speed because the
> sleeping one vanished with IBRS set. SSBD works the same way.
> 
> " SW should clear [SSBD] when enter sleep state, just as is suggested for
>   IBRS and STIBP on existing implementations"
> 
> and that document says:
> 
> "Enabling IBRS on one logical processor of a core with Intel
>  Hyper-Threading Technology may affect branch prediction on other logical
>  processors of the same core. For this reason, software should disable IBRS
>  (by clearing IA32_SPEC_CTRL.IBRS) prior to entering a sleep state (e.g.,
>  by executing HLT or MWAIT) and re-enable IBRS upon wakeup and prior to
>  executing any indirect branch."
> 
> So it's only a performance issue and not a fundamental problem to have it
> on when executing HLT/MWAIT
> 
> So we have two situations here:
> 
> 1) ssbd = on, i.e X86_FEATURE_SPEC_STORE_BYPASS_DISABLE
> 
>    There it is irrelevant because both threads have SSBD set permanentely,
>    so unsetting it on HLT/MWAIT is not going to lift the restriction for
>    the running sibling thread. And HLT/MWAIT is not going to be faster by
>    unsetting it and then setting it on wakeup again....
> 
> 2) SSBD via prctl/seccomp
> 
>    Nothing to do there, because idle task does not have TIF_SSBD set so it
>    never goes with SSBD set into HLT/MWAIT.
> 
> So I think we're good, but it would be nice if Intel folks would confirm
> that.

Yes, we have thought about turning off SSBD in the mwait path earlier. But
decided that it was unnecessary for the exact reasons Thomas mentioned.

Thanks.

Tim