[Xen-devel] Few questions about migration

["Amitayu Das" <amitayudas@xxxxxxxxx>]

Hi,

I had few questions regarding the mechanism employed by the hypervisor
for figuring out the pages modified during a pre-copy round of a
migration of a VM.  Please assume that a migration of a VM is
going on in the backdrop and a process (running inside that VM) is
accessing (reading/writing) into pages in memory.

1. The hypervisor maintains shadow page-table with *read-only"
permission for all the pages. When a page is being written by the
afore-menioned process (and assume the page is writable),  what is
the way for the hypervisor to know about it? There may be two
possibilities as mentioned below (please let me know which one is
correct):
       a. The guest-OS does not raise any
trap (as the page is writable); but, because a page was modified, he
informs the hypervisor. Is this correct?
       
       b. Following the writing, the TLB
has to be updated. Because of that the hypervisor comes to know about
the modification. Is this correct?

Put in other words, what exactly happens to let hypervisor track
page-modification with the help of shadow page-table or dirty-bitmap?

2. One may want to update TLB entries for the pages updated during a
pre-copy round in a batch for better performance? Then, is it the case
that hypervisor comes to know about all the modified pages during the
end of a pre-copy round when the updation of TLB is over? Pleaes note
that this would be possible for only those pages whose entries were
already available in TLB. Now, what'd happen if the writing process has
resulted in a TLB miss in the first place? 

Thanks in advance,
Amitayu

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