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

[harry harry]

Hi Paolo and Sean,

Thanks much for your prompt replies and clear explanations.

On Tue, Oct 13, 2020 at 2:43 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> No, the logic to find the HPA with a given HVA is the same as the
> hardware logic to translate HVA -> HPA.  That is it uses the host
> "regular" page tables, not the nested page tables.
>
> In order to translate GPA to HPA, instead, KVM does not use the nested
> page tables.

I am curious why KVM does not directly use GPAs as HVAs and leverage
nested page tables to translate HVAs (i.e., GPAs) to HPAs? Is that
because 1) the hardware logic of ``GPA -> [extended/nested page
tables] -> HPA[*]'' is different[**] from the hardware logic of ``HVA
-> [host regular page tables] -> HPA''; 2) if 1) is true, it is
natural to reuse Linux's original functionality to translate HVAs to
HPAs through regular page tables.

[*]: Here, the translation means the last step for MMU to translate a
GVA's corresponding GPA to an HPA through the extended/nested page
tables.
[**]: To my knowledge, the hardware logic of ``GPA -> [extended/nested
page tables] -> HPA'' seems to be the same as the hardware logic of
``HVA -> [host regular page tables] -> HPA''. I appreciate it if you
could point out the differences I ignored. Thanks!

Best,
Harry