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

[Christoph Egger <Christoph.Egger@xxxxxxx>]

On 10/04/12 15:23, Tim Deegan wrote:

> At 14:14 +0200 on 04 Oct (1349360091), Christoph Egger wrote:
>> 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.
> 
> Ah, I was led astray by the nestedhvm_is_n2() check.  It turns out that:
> nestedhvm_is_n2() returns 0 for guests that are in n2 but aren't
> hap-on-hap.  That's pretty confusing, and I encourage you to change it.
> 
> Anyway, I've checked in a modified version of your patch, as 
> http://xenbits.xen.org/hg/staging/xen-unstable.hg/rev/a9c84069c248
> Please check that it still does what you wanted. :)

Yes, it does. Thanks.
Please apply it to xen-4.2-testing as well.

Christoph


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