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

[harry harry]

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@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?

Thanks,
Harry