Re: [Xen-devel] arch_set_info_guest() and cr1

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Fri, 2011-09-23 at 00:31 +0100, Jeremy Fitzhardinge wrote:
> On 09/22/2011 12:32 AM, Ian Campbell wrote:
> > (old mail, I know)
> >
> > On Sat, 2011-08-13 at 03:17 +0100, Jeremy Fitzhardinge wrote:
> >> On 08/12/2011 03:01 PM, Mukesh Rathor wrote:
> >>> Ah I see it, during save/restore, it is used. 
> >>> Well, I'm trying to keep the option of using PV paging with hybrid, so 
> >>> I may need to honor that. But that's phase 2.
> >> Though it would be nice to re-enable the use of PV writable pagetables
> >> to get access to HAP, and we could do without that.
> >>
> >> Does Xen require that the user pagetable be a proper subset of the
> >> kernel pagetable?  If we can assume that and get proper ring protections
> >> in the HVM container, then we can simply ignore the user pagetable (and
> >> would have to if we want to get good syscall performance).
> > IIRC back when I did the (now completely defunct) supervisor mode kernel
> > stuff that was exactly the assumption which was made and it certainly
> > worked in practice (although "require" might be a strong term).
> 
> Well, I guess we could add ELF notes to allow a guest to say "I really
> need separate non-intersecting user/kernel pagetables" if they really
> need it.  Or repurpose auto_translated_physmap to also mean "no separate
> user/kernel pagetables required".

IIRC that's effectively one of the things which auto_translated_physmap
implies.

> Has that ever been supported for 64-bit PV guests?

In the classic patches, yes, but not in the pvops ones AFAIK.

>   My memory of the chronology is that it died as a
> feature at about the time that 64-bit support went in.
> 
>     J



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