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RE: [Xen-devel] paging mechanism clarification

  • To: "Petersson, Mats" <Mats.Petersson@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Pradeep Singh, TLS-Chennai" <pradeep_s@xxxxxx>
  • Date: Mon, 12 Mar 2007 17:04:22 +0530
  • Delivery-date: Mon, 12 Mar 2007 04:40:25 -0700
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcdkaxdIRFOb74LzRkumlKpf+mzsXwAI19lgAAJ5aFk=
  • Thread-topic: [Xen-devel] paging mechanism clarification

-----Original Message-----
From: Petersson, Mats [mailto:Mats.Petersson@xxxxxxx]
Sent: Mon 12-Mar-07 4:13 PM
To: Pradeep Singh, TLS-Chennai; xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] paging mechanism clarification

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Pradeep Singh, TLS-Chennai
> Sent: 12 March 2007 05:56
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] paging mechanism clarification
> Hi All,
> The Xen uses 2 level Paging Mechanism to resolve the Virtual
> Addresses into frame numbers from domU.The first level is
> taken care by the MMU for the domU i.e translation from
> virtual address to the physical address just like normal
> paging mechanism. The second level of translation is done by
> the Xen hypervisor.It translates the pseduo physical address
> recieved from the domU and treats it as a normal virtual
> address and finds the page frame using regualr paging mechanism.

No, not in the current model.

Does it include xen-3.0.3 also?
I hope it is true for whole xen3 series?

The paging in HVM (fully virtualized domains) is managed by the Shadow
paging, which simplified works like this:
When paging is disabled in the guest, still enable paging in the
processor and give a CR3 to the processor that points to a map of where
the guest memory is.

Mats, when paging is disabled in guest( which i guess is the case during booting of the Xen domU ), how is this related to the paging on the processor?
Even if guest is booting or paging is disabled in the guest, hypervisor should be free from this non-paging guest instance.How is paging in the hypervisor dependent on the paging on the guest?

I hope i made myself clear.
Please enlighten me :-).

When paging is enabled, we use a shadow page-table, which is essentially
that the GUEST sees one page-table, and the processor another (thanks to
the fact that the hypervisor intercepts the CR3 read/write operations,
and when CR3 is read back by the guest, we don't send back the value
it's ACTUALLY POINTING TO IN THE PROCESSOR, but the value that was set
by the guest). So there are two page-tables.

Got this well, thanks Mats :).

To make the page-table updates by the guest visible to the hypervisor,
all of the guest-page-tables are made read-only (by scanning the new CR3
value whenever one is set).

I didn't get this either well :(
sorry, but do you mean CR3 for the guest or for the processor? i hope you mean guest?

Whenever a page-fault happens, the hypervisor has "first look", and
determines if the update is for a page-table or not. If it is a
page-table update, the guest operation is emulated (in x86_emulate.c),
and the result is written to the shadow-page-table AND the

Why do we need emulation?some peculiar reason for emulating?
Do you mean to say if i am running a 32 bit domU on top of a 64 bit processor, the guest operation for updating the page table is emulated by the hypervisor.am i right?

Does this means on a x86 platform this overkill or this emulation is skipped altogether?
Please bear with me as i am an absolute Xen newbie out here :-).

guest-page-table, but in the shadow-page-table, the value is modified to
reflect the actual address in machine-space, rather than what the guest
thinks it should be.

In futuer versions of AMD processors (and I believe Intel are working on
something very similar if not the same), there will be a mode where the
processor is able to work in "nested paging mode", which means that
there are two "parallel" page-tables. The first one is the
"guest-page-table", the second one is the "host-page-table". In this
case, every lookup in the guest-page-table will be done through the
host-page-table. So we have a "simple" way to just take the
guest-page-table and translate it to machine-physical-address - with the
good thing that the host-page-table needn't change, since the pages that
the host consists of is pretty much static for the duration of the

Yes, read about about this in an article mention how Pacifica is better than VT.

Say for example, we have a guest that lives at 256-512MB. The
guest-page-table would contain, for example, a mapping for 0x12200000 ->
guest-physical 0x100000 (1MB). The host-page-table translates this to
0x10100000 because the 1MB entry in guest-address is 256+1MB in

Exactly, got this well on spot :).

[In reality, it's very likely that the guest never gets all the space in
one big chunk, but rather a few pages here and a few pages there. If
there are big chunks, we could use large pages to map those!].

Thanks a ton Mats and all.


The support for nested paging (called HAP, Hardware Assisted Paging) is
in the Unstable version of Xen since a few days back.

> And this whole 2 level paging consitutes Xen's shadow page
> tables. Right?
> Is my understanding of Xen's paging mechanism correct?or am i
> missing something?
> Thank you
> -pradeep


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