Why guest physical addresses are not the same as the corresponding host virtual addresses in QEMU/KVM? Thanks!

Sean Christopherson sean.j.christopherson at intel.com
Tue Oct 13 04:52:45 UTC 2020

On Tue, Oct 13, 2020 at 12:30:39AM -0400, harry harry wrote:
> Hi Sean,
> Thank you very much for your thorough explanations. Please see my
> inline replies as follows. Thanks!
> On Mon, Oct 12, 2020 at 12:54 PM Sean Christopherson
> <sean.j.christopherson at intel.com> wrote:
> >
> > No, the guest physical address spaces is not intrinsically tied to the host
> > virtual address spaces.  The fact that GPAs and HVAs are related in KVM is a
> > property KVM's architecture.  EPT/NPT has absolutely nothing to do with HVAs.
> >
> > As Maxim pointed out, KVM links a guest's physical address space, i.e. GPAs, to
> > the host's virtual address space, i.e. HVAs, via memslots.  For all intents and
> > purposes, this is an extra layer of address translation that is purely software
> > defined.  The memslots allow KVM to retrieve the HPA for a given GPA when
> > servicing a shadow page fault (a.k.a. EPT violation).
> >
> > When EPT is enabled, a shadow page fault due to an unmapped GPA will look like:
> >
> >  GVA -> [guest page tables] -> GPA -> EPT Violation VM-Exit
> >
> > The above walk of the guest page tables is done in hardware.  KVM then does the
> > following walks in software to retrieve the desired HPA:
> >
> >  GPA -> [memslots] -> HVA -> [host page tables] -> HPA
> Do you mean that GPAs are different from their corresponding HVAs when
> KVM does the walks (as you said above) in software?

What do you mean by "different"?  GPAs and HVAs are two completely different
address spaces.

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