Re: [PATCH] KVM: MMU: Segregate mmu pages created with different cr4.pge settings

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jan 07, 2009 at 12:19:26PM +0200, Avi Kivity wrote:
> Alexander Graf wrote:
>> Using this patch it works. But if I read it correctly, that doesn't
>> actually fix anything but only treats NPT/EPT special, which it
>> shouldn't, should it? 
>
> The patch doesn't fix the bug but is nevertheless correct.  cr4.pge only  
> matters to the mmu if using the shadow mmu; with tdp it only wastes  
> memory (and exposes the bug which you encountered).
>
> So, wrt to the bug you saw, it's a workaround, but it's also a correct  
> fix for another bug.
>
>> Maybe this actually even breaks EPT?
>>   
>
> It shouldn't.
>
>> I remember having seen a lot of CR4 hacks in svm.c when npt is enabled.
>> Maybe that is related?
>>   
>
> No.  cr4 controls the guest mmu, but with npt the guest mmu is  
> completely virtualized, so we need to ignore those bits.

Let me shoot at one direction: a shadow page with PGE bit in either
state is created. Later that shadow page is nuked (via mmu notifiers,
for example). Then set_cr4 changes base_role.pge to a different value,
and a fault creates a new shadow page and instantiates that in the tree.

Perhaps a svm_flush_tlb is required in such case, when updating a
previously valid pagetable entry? Joerg?