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

[Juergen Gross <jgross@xxxxxxxx>]

On 09/12/16 00:50, Stefano Stabellini wrote:
> 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.
> 

No, I don't think so. I believe there will be no hardware capable of
5-level paging but without VMX/SVM support. Support of PVH/HVM for such
large guests should be enough. We don't need to extend PV which we want
to get rid of in Linux anyway, no?

Juergen

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