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

Roman Kagan
2018-06-29T12:52:16Z

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.

Note that the userspace can actually do nasty things with these
VP_INDEXes, like, say, have them non-unique. We need to be resilent to
it, but don't need to optimize for it.

I think I'd rather have a warning in kvm_hv_set_msr if the VP_INDEX
being assigned is not equal to the vcpu index, and start worrying about
optimization only if this warning starts being triggered by real
hypervisor applications.

Anyway I don't see an urgent need to bloat this patchset with optimizing
this translation; it can be done separately, if needed.

Roman.