Re: [Xen-devel] Question about MMU update on HVM guest

[George Dunlap <dunlapg@xxxxxxxxx>]

On Fri, May 13, 2016 at 12:11 AM, AnhNN <ngocanh198@xxxxxxxxx> 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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