Re: [PATCH v3] x86/speculation, KVM: only IBPB for switch_mm_always_ibpb on vCPU load

[Sean Christopherson <seanjc@google.com>]

On Sat, Apr 30, 2022, Jon Kohler wrote:
> 
> > On Apr 30, 2022, at 5:50 AM, Borislav Petkov <bp@alien8.de> wrote:
> > So let me try to understand this use case: you have a guest and a bunch
> > of vCPUs which belong to it. And that guest gets switched between those
> > vCPUs and KVM does IBPB flushes between those vCPUs.
> > 
> > So either I'm missing something - which is possible - but if not, that
> > "protection" doesn't make any sense - it is all within the same guest!
> > So that existing behavior was silly to begin with so we might just as
> > well kill it.
> 
> Close, its not 1 guest with a bunch of vCPU, its a bunch of guests with
> a small amount of vCPUs, thats the small nuance here, which is one of 
> the reasons why this was hard to see from the beginning. 
> 
> AFAIK, the KVM IBPB is avoided when switching in between vCPUs
> belonging to the same vmcs/vmcb (i.e. the same guest), e.g. you could 
> have one VM highly oversubscribed to the host and you wouldn’t see
> either the KVM IBPB or the switch_mm IBPB. All good. 

No, KVM does not avoid IBPB when switching between vCPUs in a single VM.  Every
vCPU has a separate VMCS/VMCB, and so the scenario described above where a single
VM has a bunch of vCPUs running on a limited set of logical CPUs will emit IBPB
on every single switch.