RE: [PATCH v4 2/5] KVM: x86: Add IBPB support

[Van De Ven, Arjan]

> On Wed, Jan 31, 2018 at 8:55 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> > In fact this MSR can even be passed down unconditionally, since it needs
> > no save/restore and has no ill performance effect on the sibling
> > hyperthread.
> 
> I'm a bit surprised to hear that IBPB has no ill performance impact on
> the sibling hyperthread. On current CPUs, this has to flush the BTB,
> doesn't it? And since the BTB is shared between hyperthreads, doesn't
> the sibling lose all of its branch predictions?

IBPB most definitely (in current implementations) will stop both hyperthreads for
the flush.

IBPB is not a cheap operation on a system level