Re: [Qemu-devel] [RFC for-2.13 0/7] spapr: Clean up pagesize handling

[David Gibson]

On Tue, Apr 24, 2018 at 05:35:59PM +0200, Andrea Bolognani wrote:
> On Fri, 2018-04-20 at 20:21 +1000, David Gibson wrote:
> > On Fri, Apr 20, 2018 at 11:31:10AM +0200, Andrea Bolognani wrote:
> > > I'll have to look into it to be sure, but I think it should be
> > > possible for libvirt to convert a generic
> > > 
> > >   <memoryBacking>
> > >     <hugepages/>
> > >   </memoryBacking>
> > > 
> > > to a more specific
> > > 
> > >   <memoryBacking>
> > >     <hugepages>
> > >       <page size="16384" unit="KiB"/>
> > >     </hugepages>
> > >   </memoryBacking>
> > > 
> > > by figuring out the page size for the default hugepage mount,
> > > which actually sounds like a good idea regardless. Of course users
> > > user would still be able to provide the page size themselves in the
> > > first place.
> > 
> > Sounds like a good approach.
> 
> Unfortunately it seems like this is not going to be feasible, as
> POWER8 is apparently the only platform that enforces a strict
> relationship between host page size and guest page size: x86,
> aarch64 (and I have to assume POWER9 as well?) can reportedly all
> deal gracefully with guests migrating between hosts that have
> different hugepage mounts configured.

Yes, that's right.  As you guess POWER9 will also allow this.. at
least as long as the guest is in radix mode.  If the guest is in hash
mode (which includes but isn't limited to POWER8 compat mode guests)
then it will suffer from the same pagesize limitations as POWER8.

> I need to spend some more time digesting the rest of the
> information you provided, but as it stands right now I'm starting
> to think this might actually need to be its own, explicit opt-in
> knob at the libvirt level too after all :(

Poo.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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