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

From: harry harry <hiharryharryharry@gmail.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
	qemu-devel@nongnu.org, mathieu.tarral@protonmail.com,
	stefanha@redhat.com, libvir-list@redhat.com, kvm@vger.kernel.org,
	pbonzini@redhat.com
Subject: Re: Why guest physical addresses are not the same as the corresponding host virtual addresses in QEMU/KVM? Thanks!
Date: Tue, 13 Oct 2020 01:00:47 -0400	[thread overview]
Message-ID: <CA+-xGqNd37hyhAbkWxcze3YoVxY3a=_79b+ecF9+ZFCpbqcnnA@mail.gmail.com> (raw)
In-Reply-To: <20201012165428.GD26135@linux.intel.com>

BTW, I still have one more question 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
>
> 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.

Thanks,
Harry