Re: [PATCH 0/18][RFC] Nested Paging support for Nested SVM (aka NPT-Virtualization)

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Mar 12, 2010 at 09:36:41AM +0200, Avi Kivity wrote:
> On 03/11/2010 10:58 PM, Marcelo Tosatti wrote:
> >
> >>>Can't you translate l2_gpa ->  l1_gpa walking the current l1 nested
> >>>pagetable, and pass that to the kvm tdp fault path (with the correct
> >>>context setup)?
> >>If I understand your suggestion correctly, I think thats exactly whats
> >>done in the patches. Some words about the design:
> >>
> >>For nested-nested we need to shadow the l1-nested-ptable on the host.
> >>This is done using the vcpu->arch.mmu context which holds the l1 paging
> >>modes while the l2 is running. On a npt-fault from the l2 we just
> >>instrument the shadow-ptable code. This is the common case. because it
> >>happens all the time while the l2 is running.
> >OK, makes sense now, I was missing the fact that the l1-nested-ptable
> >needs to be shadowed and l1 translations to it must be write protected.
> 
> Shadow converts (gva -> gpa -> hpa) to (gva -> hpa) or (ngpa -> gpa
> -> hpa) to (ngpa -> hpa) equally well.  In the second case npt still
> does (ngva -> ngpa).
> 
> >You should disable out of sync shadow so that l1 guest writes to
> >l1-nested-ptables always trap.
> 
> Why?  The guest is under obligation to flush the tlb if it writes to
> a page table, and we will resync on that tlb flush.

The guests hypervisor will not flush the tlb with invlpg for updates of
its NPT pagetables. It'll create a new ASID, and KVM will not trap
that.

> Unsync makes just as much sense for nnpt.  Think of khugepaged in
> the guest eating a page table and spitting out a PDE.
> 
> >And in the trap case, you'd have to
> >invalidate l2 shadow pagetable entries that used the (now obsolete)
> >l1-nested-ptable entry. Does that happen automatically?
> 
> What do you mean by 'l2 shadow ptable entries'?  There are the
> guest's page tables (ordinary direct mapped, unless the guest's
> guest is also running an npt-enabled hypervisor), and the host page
> tables.  When the guest writes to each page table, we invalidate the
> shadows.

With 'l2 shadow ptable entries' i mean the shadow pagetables that
translate GPA-L2 -> HPA.