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Re: [Xen-devel] [PATCH] nestedsvm: fix paging mode

On 10/04/12 12:37, Tim Deegan wrote:

> At 15:50 +0200 on 01 Oct (1349106630), Christoph Egger wrote:
>> On 09/27/12 16:53, Tim Deegan wrote:
>>> At 16:01 +0200 on 21 Sep (1348243291), Christoph Egger wrote:
>>>> On VMRUN and VMEXIT emulation update the paging mode
>>>> for Shadow-on-Nested. This allows Xen to walk the
>>>> l1 hypervisors shadow page table correctly.
>>>> Problem found with 64bit Win7 and 32bit XPMode where
>>>> Win7 switches forth and back between long mode and
>>>> PAE legacy pagetables.
>>>> Signed-off-by: Christoph Egger <Christoph.Egger@xxxxxxx>
>>> Don't you have to do this in other cases as well?  I think that
>>> shadow-on-shadow might need it, at least.
>> It is needed for all cases where the l1 guest does shadow paging.
>> This includes: Shadow-on-Nested and Shadow-on-Shadow.
> I've looked more closely at this and now I'm more confused. :)
> Hap-on-hap seems to be OK without it because the special case in
> paging_gva_to_gfn() does the right thing, using the nestedmode's pt
> walker.
> Why is that not good enough for shadow-on-hap?  Is there another path
> that does unguarded pt walks?  If so:
>  - why is that path not a problem for hap-on-hap; and
>  - shouldn't that be handled the same way, i.e. either handle everything
>    at lookup time, like paging_gva_to_gfn() does, or handle everything
>    by switching modes at VMRUN/EXIT?

If the l1 guest does not do nested paging then Xen doesn't use the
nestedmode's pt walker.


> Shadow-on-shadow could potentially be handled the same way as the other
> configurations, by extending the special case in paging_gva_to_gfn(),
> but I suspect that a mode switch on VMRUN/EXIT is more likely to Just
> Work there.
> Tim.

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