From mboxrd@z Thu Jan  1 00:00:00 1970
From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Subject: Re: How KVM sync guest page table with corresponding shadow page
 table?
Date: Tue, 14 Aug 2012 17:23:01 +0800
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On 08/14/2012 11:54 AM, =E9=99=B3=E9=9F=8B=E4=BB=BB (Wei-Ren Chen) wrot=
e:
> Hi all,
>=20
>   If guest modify its guest page table entry, KVM should trap that ev=
ent,
> emulate what guest trying to do, and sync with the corresponding shad=
ow
> page table entry. Is my understanding correct? If so, does KVM use
> something like mprotect to make guest page table read-only, or there =
is
> another way?=20

It is controlled by shadow page table, guest-page-tables are write-prot=
ected
on shadow pages (the W bit on PTE is cleared).

There has a special case, called unsync shadow page, if the page only u=
sed
as guest page structure on the lowest level (level =3D 1), we allow it =
to be
writable, it will be sync-ed when the guest flush the tlb (e.g: CR3 rel=
oad,
invlpg...) because according to x86 TLB rules, it needs to flush tlb to=
 apply
the change.

> Besides, how KVM know what guest try to write into the
> guest page table entry?

In the normal case, guest writes its page table will generate #PF since=
 the
page is write-protected as we mention above.

=46or the special case (unsync shadow page), we can happily write its p=
age
table without #PF.