RE: [patch RFC 5/5] x86/speculation: Add basic speculation control code

[Van De Ven, Arjan]

> So here is the simple list of questions all to be answered with YES or
> NO. I don't want to see any of the 'but, though ...'. We all know by now
> that it's CPU dependent and slow and whatever and that IBRS_ATT will be in
> future CPUs. So get your act together and tell a clear YES or NO.

I will answer for current cpus, so those not having IBRS_ATT
> 
> 1) Does IBRS=1 when set once act as a set-and-forget option ?

NO

 
>  1a) If the answer to #1 is yes, is it more secure than toggling it?

NO
>
>  1b) If the answer to #1 is yes, is retpoline required ?

NO - Once you have IBRS done PROPERLY (toggled) you don't need retpoline
> 
>  1c) If the answer to #1 is yes, is RSB stuffing required ?

Only for the VM exit case

> 
> 2) Does toggle mode of IBRS require retpoline ?

NO

> 
> 3) Does toggle mode of IBRS require RSB stuffing ?

Only for the VM exit case
> 
> 4) Exist CPUs which require IBRS to be selected automatically ?

I do not understand your question exactly