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

[Juergen Gross <jgross@xxxxxxxx>]

On 09/12/16 10:59, Jan Beulich wrote:
>>>> On 08.12.16 at 18:22, <andrew.cooper3@xxxxxxxxxx> 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.
> 
> Right; a first question though would be whether 5-level support
> would be a build time selection (just like 32-bit PAE was long ago),
> or runtime determined.

Guessing you mean Linux kernel here: the intention is to have one kernel
being capable to use 4- or 5-level paging in the end. Current patches
don't support this, they are using a Kconfig option to select the number
of page table levels.

>> 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.
> 
> I agree, but we may face resistance from (Linux and other) kernel
> folks.
> 
>> However, a big question is whether any of this effort is worth doing, in
>> the light of PVH.
> 
> Much depends, I think, on how quickly this becomes a fully
> supported feature.

"this" refers to PVH, I guess?


Juergen


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