Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user space protection mode

[Tim Chen]

On 11/26/2018 01:52 PM, Lendacky, Thomas wrote:
> On 11/26/2018 02:58 PM, Thomas Gleixner wrote:
>> On Mon, 26 Nov 2018, Andrea Arcangeli wrote:
>>
>>> Hello,
>>>
>>> On Sun, Nov 25, 2018 at 11:28:59PM +0100, Thomas Gleixner wrote:
>>>> Indeed. Just checked the documentation again, it's also not clear whether
>>>> IBPB is required if STIPB is in use.
>>>
>>> I tried to ask this question too earlier:
>>>
>>> https://lkml.kernel.org/r/20181119234528.GJ29258@xxxxxxxxxx
>>>
>>> If the BTB mistraining in SECCOMP context with STIBP set in SPEC_CTRL,
>>> can still influence the hyperthreading sibling after STIBP is cleared,
>>> IBPB is needed before clearing STIBP. Otherwise it's not. Unless told
>>> otherwise, it'd be safe to assume IBPB is needed in such case.
>>
>> IBPB is still issued. I won't change that before we have clarification.
> 
> From an AMD standpoint, we recommend still issuing the IBPB.
> 

Yes, our Intel HW architect also recommends still issuing the IBPB.  We're now
getting approval for some additional explanations of STIBP.  Those additional
explanations should help clarify things.

Thanks.

Tim