Re: [PATCH v5 1/9] KVM: MMU: fix forgot reserved bits check in speculativepath

[Avi Kivity]

On 07/12/2010 05:37 AM, Xiao Guangrong wrote:
>    
> > > +    if (is_rsvd_bits_set(vcpu, gentry, PT_PAGE_TABLE_LEVEL))
> > > +        gentry = 0;
> > > +
> > >
> > >        
> > That only works if the gpte is for the same mode as the current vcpu mmu
> > mode.  In some cases it is too strict (vcpu in pae mode writing a 32-bit
> > gpte), which is not too bad, in some cases it is too permissive (vcpu in
> > nonpae mode writing a pae gpte).
> >
> >      
> Avi, thanks for your review.
>
> Do you mean that the VM has many different mode vcpu? For example, both
> nonpae vcpu and pae vcpu are running in one VM? I forgot to consider this
> case.
>    

Yes.  This happens while the guest brings up other vcpus, and when using 
nested virtualization.

> > (once upon a time mixed modes were rare, only on OS setup, but with
> > nested virt they happen all the time).
> >      
> I'm afraid it's still has problem, it will cause access corruption:
> 1: if nonpae vcpu write pae gpte, it will miss NX bit
> 2: if pae vcpu write nonpae gpte, it will add NX bit that over gpte's width
>
> How about only update the shadow page which has the same pae set with the written
> vcpu? Just like this:
>
> @@ -3000,6 +3000,10 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>                  while (npte--) {
>                          entry = *spte;
>                          mmu_pte_write_zap_pte(vcpu, sp, spte);
> +
> +                       if (!!is_pae(vcpu) != sp->role.cr4_pae)
> +                               continue;
> +
>    

Not enough, one vcpu can have nx set while the other has it reset, etc.

--
error compiling committee.c: too many arguments to function

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