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


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