Re: [Xen-devel] [XenSummit 2017] Notes from the 5-level-paging session

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 7/17/2017 6:53 PM, Juergen Gross wrote:
> Hey,
>
> I took a few notes at the 5-level-paging session at the summit.
> I hope there isn't any major stuff missing...
>
> Participants (at least naming the active ones): Andrew Cooper,
> Jan Beulich, Yu Zhang and myself (the list is just from my memory).
>
> The following topics have been discussed in the session:
>
>
> 1. Do we need support for 5-level-paging PV guests?
>
> There is no urgent need for 5-level-paging PV guests for the
> following reasons:
>
> - Guests >64TB (which is the upper limit for 4-level-paging Linux)
>    can be PVH or HVM.
>
> - A 5-level-paging host supports up to 4 PB physical memory. A
>    4-level-paging PV-Dom0 can support that theoretically: the M2P map
>    for 4 PB memory needs 8 TB space, which just fits into the hypervisor
>    reserved memory area in the Linux kernel. Any other hypervisor data
>    and/or code can live in the additionally available virtual space of
>    the 5-level-paging mode.
>
> There was agreement we don't need support of 5-level-paging PV guests
> right now. There is a need, however, to support 4-level-paging PV
> guests located anywhere in the 52-bit physical space of a 5-level-paging
> host (right now they would have to be in the bottom 64 TB as the Linux
> kernel is masking away any MFN bit above 64 TB). I will send patches to
> support this.
>
>
> 2. Do we need 5-level-paging shadow mode support?
>
> While strictly required for PV guests only and no 5-level-paging PV
> guests are to be supported, we will need 5-level-paging shadow mode in
> the long run. This is necessary because even for a 4-level-paging PV
> guest (or a 32-bit PV guest) the processor will run in 5-level-paging
> mode on a huge host as switching between the paging modes is rather
> complicated and should be avoided. It is much easier to run shadow
> mode for the whole page table tree instead for two subtrees only.
>
> OTOH the first step when implementing 5-level-paging in the hypervisor
> doesn't require shadow mode to be working, so it can be omitted in the
> beginning.
>
>
> 3. Is it possible to support 5-level-paging in Xen via a specific
>     binary for the first step?
>
> Yu Zhang asked for implementing 5-level-paging via a Kconfig option
> instead of dynamical switching at boot time for the first prototype.
> This request was accepted in order to reduce the complexity of the
> initial patches. Boot time switching should be available for the
> final solution, though.
>
>
> I hope I didn't miss anything.
Thanks a lot for the your help and for the summary, Juergen.
And I really need to say thank you for quite a lot people who joined 
this discussion. It's quite
enlightening. :)

One thing I can recall is about the wr{fs,gs}base for pv guest. IIRC, 
our agreement is to turn off
the FSGSBASE in cr4 for PV guests and try to emulate the rd{fs,gs}base 
and wr{fs,gs}base in the
#UD handler.

But please correct me if I misunderstood. :)

B.R.
Yu


> Juergen
>
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