Re: [PATCH 23/35] x86/speculation: Add basic speculation control code

[Paolo Bonzini]

On 18/01/2018 18:08, Dave Hansen wrote:
> On 01/18/2018 08:37 AM, Josh Poimboeuf wrote:
>>>
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -3932,6 +3932,7 @@
>>>  			retpoline	  - replace indirect branches
>>>  			retpoline,generic - google's original retpoline
>>>  			retpoline,amd     - AMD-specific minimal thunk
>>> +			ibrs		  - Intel: Indirect Branch Restricted Speculation
>> Are there plans to add spectre_v2=ibrs_always to prevent SMT-based
>> attacks?
> 
> What does "ibrs_always" mean to you?
> 
> There is a second bit in the MSR (STIBP) that is intended to keep
> hyperthreads from influencing each-other.  That is behavior is implicit
> when IBRS is enabled.

Yeah, I think we should have a mode to always leave that enabled, or
always set IBRS=1.

> I think ibrs_always *should* probably be kept to refer to the future
> CPUs that can safely leave IBRS enabled all the time.

Is that "safely" or "without throwing performance down the drain"?

Does "always IBRS=1" *hinder* the mitigation on existing processor, as
long as you reset IBRS=1 on kernel entry and vmexit?  Or is it just slow?

Paolo