Re: [Qemu-devel] [libvirt] QEMU -M nvdimm=on and hotplug

[Igor Mammedov]

On Thu, 14 Sep 2017 13:51:45 +0200
Michal Privoznik <address@hidden> wrote:

> On 09/14/2017 02:33 AM, Haozhong Zhang wrote:
> > On 09/13/17 17:28 +0200, Michal Privoznik wrote:  
> >>
> >> BTW: I ran a migration from no nvdimm qemu to one that had -M nvdimm=on
> >> and guest migrated happily. So looks like guest ABI is stable (or at
> >> least stable enough not to crash). But since ACPI table is changed I
> >> doubt that.  
> > 
> > One example that may cause trouble is that
> > 1/ Guest OS got a pointer to an ACPI table A on the source host (w/o 
> > nvdimm=on)
> > 2/ After migrating to the destination host (w/ nvdimm=on), the
> >    location of ACPI table A is occupied by NFIT. If guest OS tries to
> >    access ACPI table A via the pointer in step 1/, then it will access
> >    the wrong table.
> >   
> 
> Ah, you're right. So it a guest ABI breakage to add nvdimm=on. IOW,
> libvirt can't safely add that onto command line. Well we could for
> freshly started guest and not those which are just being migrated. But
> that increases attack surface so it's not safe either. Okay, I'll stick
> with the latest proposal (expose this in domain XML and let users turn
> it on if they want to).
also note that it depends on memory hotplug being enabled '-m xxx,slots+maxmem'