Re: [PATCH v3 0/4] KVM: Expose speculation control feature to guests

[Dave Hansen]

On 01/30/2018 04:16 PM, Paolo Bonzini wrote:
> On 30/01/2018 18:48, Raj, Ashok wrote:
>>> Certainly not every vmexit!  But doing it on every userspace vmexit and
>>> every sched_out would not be *that* bad.
>> Right.. agreed. We discussed the different scenarios that doing IBPB
>> on VMexit would help, and decided its really not required on every exit. 
>>
>> One obvious case is when there is a VMexit and return back to Qemu
>> process (witout a real context switch) do we need that to be 
>> protected from any poisoned BTB from guest?
> If the host is using retpolines, then some kind of barrier is needed.  I
> don't know if the full PRED_CMD barrier is needed, or two IBRS=1/IBRS=0
> writes back-to-back are enough.

I think the spec is pretty clear here: protection is only provided
*while* IBRS=1.  Once it goes back to 0, all bets are off.