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

[Paolo Bonzini]

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.

If the host is using IBRS, then writing IBRS=1 at vmexit has established
a barrier from the less privileged VMX guest environment.

Paolo

> If Qemu is protected by !dumpable/retpoline that should give that gaurantee.
> We do VM->VM IBPB at vmload() time that should provide that gaurantee.