Question about MMU update on HVM guest

Question about MMU update on HVM guest

[AnhNN]

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Hi,

I have some questions about MMU update operation.
I add some logging in function *do_mmu_update* (in file
/xen/arch/x86/mm.c), and start Windows 7 32 bit HVM guest.
After guest started, I look at log and see that MMU update has been called
with every page of guest, with *pt_ower = 0* and *pg_owner = 1*. And with
every page, MMU update called 2 times. The first time, * page->count_info =
0x8000000000000002* after MMU update, but after that it decrease to
*0x8000000000000001* in a different function. At the second time,
*page->count_info
= 0x8000000000000002* after MMU update, and keep that value forever.

So the question is, why domain 0 have a reference to every pages of HVM
guest ?
And why in the second time of MMU update, count_info doesn't decrease to
*0x8000000000000001* ?

Thanks & Best Regards.

-- 
Nguyễn Ngọc Anh
Student - Course 53
Department of Computer Engineering - School of Information and
Communication Technology
Hanoi University Of Science And Technology.
Mobile: 0973402195
Email: ngocanh198@gmail.com <hiepnp1990@gmail.com>
Y!M: pancuro
Skype: ngocanh19890

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Re: Question about MMU update on HVM guest

[George Dunlap]

On Fri, May 13, 2016 at 12:11 AM, AnhNN <ngocanh198@gmail.com> wrote:
> Hi,
>
> I have some questions about MMU update operation.
> I add some logging in function do_mmu_update (in file /xen/arch/x86/mm.c),
> and start Windows 7 32 bit HVM guest.
> After guest started, I look at log and see that MMU update has been called
> with every page of guest, with pt_ower = 0 and pg_owner = 1. And with every
> page, MMU update called 2 times. The first time, page->count_info =
> 0x8000000000000002 after MMU update, but after that it decrease to
> 0x8000000000000001 in a different function. At the second time,
> page->count_info = 0x8000000000000002 after MMU update, and keep that value
> forever.
>
> So the question is, why domain 0 have a reference to every pages of HVM
> guest ?
> And why in the second time of MMU update, count_info doesn't decrease to
> 0x8000000000000001 ?

I think your best best for understanding where the references are
coming from is to look at the get_page / put_page.

One question: is your guest running in HVM mode, or in shadow mode?

 -George

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