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

harry harry hiharryharryharry at gmail.com
Tue Oct 13 05:00:47 UTC 2020

BTW, I still have one more question 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
> KVM then takes the resulting HPA and shoves it into KVM's shadow page tables,
> or when TDP is enabled, the EPT/NPT page tables.  When the guest is run with
> TDP enabled, GVA->HPA translations look like the following, with all walks done
> in hardware.
>  GVA -> [guest page tables] -> GPA -> [extended/nested page tables] -> HPA

If I understand correctly, the hardware logic of MMU to walk ``GPA ->
[extended/nested page tables] -> HPA''[1] should be the same as ``HVA
-> [host page tables] -> HPA"[2]. If not true, how does KVM find the
correct HPAs when there are EPT violations?

[1] Please note that this hardware walk is the last step, which only
translates the guest physical address to the host physical address
through the four-level nested page table.
[2] Please note that this hardware walk assumes translating the HVA to
the HPA without virtualization involvement.


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