page table pages with checkpoint functionality

page table pages with checkpoint functionality

[박은병]

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Hi, all

I am analyzing the xen source about checkpoint functionality
(tools/libxc/xc_domain_save.c, xc_domain_restore.c)

during saving page table pages to disk or another machine( in the case of
live migration), those pages should be canonicalized,
which means that references to actual mfns are replaced with references to
the corresponding pfns

I can understand that, but I could see the another point that I can't
understand

After printing out the page table entries, I found that most of page table
entries has _PAGE_DIRTY, _PAGE_ACCESSED, _PAGE_RW bit.

As soon as guest OS's booting is completed, I checkpointed the guest OS.
(command is xm save [domainU])

L1 pagetable pages:341
L2 pagetable pages:71
L3 pagetable pages:19
L4 pagetable pages:0
(the number of L4 is 0 because of I have used PAE enabled xen)

As futher investigating, L1 page table entries,

page table entries that have _PAGE_DIRTY :131951
page table entries that have _PAGE_ACCESSED :133960
page table entries that have _PAGE_RW :129787

In PAE enabled level 3 paging, each L1 page table has 512 entries, so total
entries 341*512 = 174592
70~80% ptes have those bits

I make sense of many _PAGE_ACCESSED bits.
but, why do those pages have another 2 bits??

Thanks.
-- 
Eunbyung Park

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