Re: [Xen-devel] Future support of 5-level paging in Xen:wq

[Stefano Stabellini <stefano@xxxxxxxxxxx>]

On Thu, 8 Dec 2016, Andrew Cooper wrote:
> On 08/12/2016 19:18, Stefano Stabellini wrote:
> > On Thu, 8 Dec 2016, Andrew Cooper wrote:
> >> On 08/12/16 16:46, Juergen Gross wrote:
> >>> The first round of (very preliminary) patches for supporting the new
> >>> 5-level paging of future Intel x86 processors [1] has been posted to
> >>> lkml:
> >>>
> >>> https://lkml.org/lkml/2016/12/8/378
> >>>
> >>> An explicit note has been added: "CONFIG_XEN is broken." and
> >>> "I would appreciate help with the code."
> >>>
> >>> I think we should start a discussion what we want to do in future:
> >>>
> >>> - are we going to support 5-level paging for PV guests?
> >>> - do we limit 5-level paging to PVH and HVM?
> >> The 64bit PV ABI has 16TB of virtual address space just above the upper
> >> 48-canonical boundary.
> >>
> >> Were Xen to support 5-level PV guests, we'd either leave the PV guest
> >> kernel with exactly the same amount of higher half space as it currently
> >> has, or we'd have to recompile Xen as properly position-independent and
> >> use a different virtual range in different paging mode.
> >>
> >> Another pain point is the quantity of virtual address space handed away
> >> in the ABI.  We currently had 97% of the virtual address space away to
> >> 64bit PV guests, and frankly this is too much.  This is the wrong way
> >> around when Xen has more management to do than the guest.  If we were to
> >> go along the 5-level PV guests route, I will insist that there is a
> >> rather more even split of virtual address space baked into the ABI.
> >>
> >> However, a big question is whether any of this effort is worth doing, in
> >> the light of PVH.
> > With my Aporeto hat on, I'll say that given the overwhelming amount of
> > hardware available out there without virtualization support, this work
> > is worth doing. I am referring to all the public cloud virtual machines,
> > which can support Xen PV guests but cannot support PVH guests.
> 
> Why is Xen supporting 5-level guests useful for running in a PV cloud
> VM?  Xen doesn't run PV.
> 
> I am not suggesting that we avoid adding 5-level support to Xen.  We
> should absolutely do that.  The question is only whether we extend the
> PV ABI to support 5-level PV guests.  Conceptually, its very easy to
> have a 5-level Xen but only supporting 4-level PV guests.
> 
> VT-x and SVM date from 2005/2006 and are now 10 years old.  I would be
> surprised if you would find much hardware of this age in any cloud; you
> can't by anything that old, and support contracts have probably run out
> if you have owned that hardware for 10 years.

I am thinking that in a couple of years, we might already find VMs so
large that to use all the memory in a nested virt scenario, we need
5-level PV abi support.

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