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From: harry harry <hiharryharryharry@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
	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
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 16:36:25 -0400	[thread overview]
Message-ID: <CA+-xGqOMKRh+_5vYXeLOiGnTMw4L_gUccqdQ+HGSOzuTosp6tw@mail.gmail.com> (raw)
In-Reply-To: <a163c2d8-d8a1-dc03-6230-a2e104e3b039@redhat.com>

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

WARNING: multiple messages have this Message-ID (diff)
From: harry harry <hiharryharryharry@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
	 Sean Christopherson <sean.j.christopherson@intel.com>
Cc: kvm@vger.kernel.org, libvir-list@redhat.com,
	qemu-devel@nongnu.org, Maxim Levitsky <mlevitsk@redhat.com>,
	mathieu.tarral@protonmail.com, stefanha@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 16:36:25 -0400	[thread overview]
Message-ID: <CA+-xGqOMKRh+_5vYXeLOiGnTMw4L_gUccqdQ+HGSOzuTosp6tw@mail.gmail.com> (raw)
In-Reply-To: <a163c2d8-d8a1-dc03-6230-a2e104e3b039@redhat.com>

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


  reply	other threads:[~2020-10-13 20:36 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-11  5:26 Why guest physical addresses are not the same as the corresponding host virtual addresses in QEMU/KVM? Thanks! harry harry
2020-10-11  5:26 ` harry harry
2020-10-11  7:29 ` Maxim Levitsky
2020-10-11  7:29   ` Maxim Levitsky
2020-10-11 14:11   ` harry harry
2020-10-11 14:11     ` harry harry
2020-10-12 16:54     ` Sean Christopherson
2020-10-12 16:54       ` Sean Christopherson
2020-10-13  4:30       ` harry harry
2020-10-13  4:30         ` harry harry
2020-10-13  4:52         ` Sean Christopherson
2020-10-13  4:52           ` Sean Christopherson
2020-10-13  5:33           ` harry harry
2020-10-13  5:46             ` harry harry
2020-10-13  6:43               ` Paolo Bonzini
2020-10-13  6:43                 ` Paolo Bonzini
2020-10-13 20:36                 ` harry harry [this message]
2020-10-13 20:36                   ` harry harry
2020-10-14  8:27                   ` Paolo Bonzini
2020-10-14  8:27                     ` Paolo Bonzini
2020-10-14  8:29                   ` Maxim Levitsky
2020-10-14  8:29                     ` Maxim Levitsky
2020-10-15  3:45                     ` harry harry
2020-10-15  3:45                       ` harry harry
2020-10-13  7:03             ` Sean Christopherson
2020-10-13  7:03               ` Sean Christopherson
2020-10-13 22:40               ` harry harry
2020-10-13 22:40                 ` harry harry
2020-10-14  8:28                 ` Paolo Bonzini
2020-10-14  8:28                   ` Paolo Bonzini
2020-10-15  3:43                   ` harry harry
2020-10-15  3:43                     ` harry harry
2020-10-13  5:00       ` harry harry
2020-10-13  5:00         ` harry harry

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