Re: [PATCH v2 2/5] KVM: x86: hyperv: introduce vp_index_to_vcpu_idx mapping

[Roman Kagan]

On Fri, Jun 29, 2018 at 05:25:56PM +0200, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
> 
> > On Fri, Jun 29, 2018 at 03:10:14PM +0200, Vitaly Kuznetsov wrote:
> >> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
> >> 
> >> > On Fri, Jun 29, 2018 at 01:37:44PM +0200, Vitaly Kuznetsov wrote:
> >> >> The problem we're trying to solve here is: with PV TLB flush and IPI we
> >> >> need to walk through the supplied list of VP_INDEXes and get VCPU
> >> >> ids. Usually they match. But in case they don't [...]
> >> >
> >> > Why wouldn't they *in practice*?  Only if the userspace wanted to be
> >> > funny and assigned VP_INDEXes randomly?  I'm not sure we need to
> >> > optimize for this case.
> >> 
> >> Can someone please remind me why we allow userspace to change it in the
> >> first place?
> >
> > I can ;)
> >
> > We used not to, and reported KVM's vcpu index as the VP_INDEX.  However,
> > later we realized that VP_INDEX needed to be persistent across
> > migrations and otherwise also known to userspace.  Relying on the kernel
> > to always initialize its indices in the same order was unacceptable, and
> > we came up with no better way of synchronizing VP_INDEX between the
> > userspace and the kernel than to let the former to set it explicitly.
> >
> > However, this is basically a future-proofing feature; in practice, both
> > QEMU and KVM initialize their indices in the same order.
> 
> 
> Thanks!
> 
> But in the theoretical case when these indices start to differ after
> migration, users will notice a slowdown which will be hard to explain,
> right?

That's exactly why I suggested a warning on VP_INDEX != vcpu index in
kvm_hv_set_msr.

> Does it justify the need for vp_idx_to_vcpu_idx?

I'd personally prefer being pointed at a scenario where this becomes
relevant first.

Roman.