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

["Li, Liang Z" <liang.z.li@xxxxxxxxx>]

> Subject: RE: [Xen-devel] Future support of 5-level paging in Xen:wq
> 
> > From: Li, Liang Z
> > Sent: Tuesday, December 13, 2016 11:58 AM
> >
> > Hi All,
> >
> > We are now working on enabling 5 level paging & 5 level EPT for XEN.
> > We need the community's opinions about the following aspects:
> >
> > 1.  Should we enable 5 level paging for PV guest ? (You are
> > discussing) 2.  Should we make the 5 level paging and 4 level paging
> supported with one binary?
> > 3.  When should the 5 level EPT be enabled?
> 
> Isn't it as usual i.e. let's enable once the spec is published? Or could you
> elaborate any concern of asking this question?
> 

The concern is the performance vs complexity . Walk the 5 level EPT is slower 
than the 4 level EPT because
 It needs to access the memory one more time.
We can chose to enable 5 level EPT as long as the CPU support, or only when 
it's needed, e.g.
 to support a guest with huge amount of  memory(beyond 48bit physical address 
can support),
or when the host has a small amount of memory present.

The former is simple and straight forward, and the latter is better for 
performance but add extra
mechanism  to choose the proper EPT level used. Which one to use may dependent 
on the performance
gap.

Thanks!
Liang 

> > 4. Should we extend shadow page to support 5 level paging guest?
> >
> 
> I don't think so. EPT has been there for so many years. We can assume EPT
> available on new platform which supports 5-level.
> 
> Thanks
> Kevin
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