Re: How KVM sync guest page table with corresponding shadow page table?

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 08/14/2012 11:54 AM, 陳韋任 (Wei-Ren Chen) wrote:
> Hi all,
> 
>   If guest modify its guest page table entry, KVM should trap that event,
> emulate what guest trying to do, and sync with the corresponding shadow
> 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? 

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

There has a special case, called unsync shadow page, if the page only used
as guest page structure on the lowest level (level = 1), we allow it to be
writable, it will be sync-ed when the guest flush the tlb (e.g: CR3 reload,
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.

For the special case (unsync shadow page), we can happily write its page
table without #PF.