Re: [PATCH] KVM: MMU: Replace role.glevels with role.cr4_pae

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Apr 14, 2010 at 07:32:12PM +0300, Avi Kivity wrote:
> On 04/14/2010 07:20 PM, Avi Kivity wrote:
> >There is no real distinction between glevels=3 and glevels=4; both have
> >exactly the same format and the code is treated exactly the same way.  Drop
> >role.glevels and replace is with role.cr4_pae (which is meaningful).  This
> >simplifies the code a bit.
> >
> >As a side effect, it allows sharing shadow page tables between pae and
> >longmode guest page tables at the same guest page.
> 
> >
> >  static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> >  {
> >-	if (sp->role.glevels != vcpu->arch.mmu.root_level) {
> >+	if (sp->role.cr4_pae != !!is_pae(vcpu)) {
> >  		kvm_mmu_zap_page(vcpu->kvm, sp);
> >  		return 1;
> >  	}
> 
> This bit confuses me a little.  Why is it needed?  It will never hit
> from mmu_sync_children(), and as for kvm_mmu_get_page(), it will
> simply zap unrelated pages?

kvm_mmu_get_page is write protecting a gfn. If there's shadow for a
different role, and its unsync, it needs to be synchronized.

Perhaps it could call the appropriate _sync_page version instead
of zapping, similar to mmu_pte_write_new_pte.

> Is it related to the restriction that we can only unsync if we have
> just one shadow page for a gfn?  That's somewhat artificial (and
> hurts nonpae guests, and guests with linear page tables).

If gfn is shadowed at PMD or higher level, you can't unsync the PTE 
shadow.