RE: [ndctl PATCH v2 4/4] ndctl, monitor: support NVDIMM_FAMILY_HYPERV

[Dexuan Cui <decui-0li6OtcxBFHby3iVrkZq2A@public.gmane.org>]

> From: qi.fuli-LMvhtfratI1BDgjK7y7TUQ@public.gmane.org <qi.fuli-LMvhtfratI1BDgjK7y7TUQ@public.gmane.org>
> Sent: Sunday, February 24, 2019 8:48 PM
> To: Dexuan Cui <decui-0li6OtcxBFHby3iVrkZq2A@public.gmane.org>; Dave Jiang <dave.jiang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>;
> Vishal Verma <vishal.l.verma-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>; Dan Williams
> <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>; linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org; Michael Kelley
> <mikelley-0li6OtcxBFHby3iVrkZq2A@public.gmane.org>; Johannes Thumshirn <jthumshirn-l3A5Bk7waGM@public.gmane.org>
> Subject: RE: [ndctl PATCH v2 4/4] ndctl, monitor: support
> NVDIMM_FAMILY_HYPERV
> 
> >
> > Hi Qi,
> > Generally speaking, I agree with you, but here the fact is that we can only
> monitor this
> > one single event "dimm-health-state" for Hyper-V Virtual NVDIMM, and the
> other
> > events are meaningless for Hyper-V Virtual NVDIMM, as they're not
> supported by
> > Hyper-V Virtual NVDIMM.
> >
> > So, if the user specifies more events than just "dimm-health-state", or the
> user doesn't
> > specify "dimm-health-state", it's reasonable to force monitor.event_flags to
> equal to
> > ND_EVENT_HEALTH_STATE, and I explicitly print a warning.
> >
> > The line "monitor.event_flags = ND_EVENT_HEALTH_STATE" only runs in a
> Linux VM
> > running on Hyper-V. It can't adversely affect bare metal cases. In a Linux VM
> on
> > Hyper-V, all the NVDIMMs appearing in the VM are Hyper-V Virtual
> NVDIMMs.
> >
> > A physical NVDIMM can't be directly passed through to a VM and the
> hypervisors (e.g.,
> > Xen, KVM, Hyper-V, Qemu, etc.) have to virtualize it to make it available to a
> VM. In a
> > VM, the NVDIMM_FAMILY is NVDIMM_FAMILY_HYPERV only when the VM
> runs on
> > Hyper-V.
> >
> > So I think we should be good. :-)
> >
> > Thanks,
> > -- Dexuan
> 
> Hi Dexuan,
> 
> Thanks for your explanation.
> I think at least we should document it into man page, so that the users could
> know that their options may be modified by monitor.

Good suggestion! After the patch receives more comments and/or after it's 
accepted, I'll add a new patch for the man page:
Documentation/ndctl/ndctl-monitor.txt.

> Though I have a little concern about that there may be other events for
> Hyper-V Virtual NVDIMM can be monitored in future...
>  Qi

As far as I know, that's unlikely, at least in the foreseeable future. :-)

The VM is supposed to see a virtual NVDIMM, for which the hypervisor only
emulates the minimal necessary info, based on the physical NVDIMM's state.

Thanks,
--Dexuan