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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

> Of course even the largest virtual machine today (2TB on Amazon AFAIK)
> is not close to reaching the current memory limit, but it's just a
> matter of time.

/me things Oracle will have something to say about this.  I'm sure there
was talk about VMs larger than this at previous hackathons.  XenServer
functions (ish, so long as you don't migrate) with 6TB VMs, although
starting and shutting them down feels like treacle.

~Andrew

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