Re: [Xen-devel] Questions about sh_prefetch and log_dirty

[David Knight <dongwei.net@xxxxxxxxxx>]

Thank you for your reply.
> Hi,
>
> At 17:44 +0100 on 23 Jun (1245779059), David Knight wrote:
>> I've been working on a project which is to minimize the
>> overhead of page_fault when PV is in log_dirty mode. I
>> wrote a function "sh_prebuild", it's very much similar
>> to sh_prefetch(), the major difference between them is:
>> I set the "ft" as "ft_demand_write" rather than
>> "ft_prefetch" when I call l1e_propagate_from_guest().
>
> I'm not sure that's a good idea.  The point of log-dirty is that it lets
> the migration tool resend only the dirtied pages, and although your
> change will avoid some page faults, it will increase the number of
> frames that have to be retransmitted.
Yes, but according to our locality statistics. Marking the nearest 3
or 4 pages as writable will
not be a big problem.
>
> Also, just setting ft_demand_write won't always do what you want: in
> _sh_propagate, if the guest PTE's _PAGE_DIRTY bit isn't set it will mask
> out _PAGE_RW anyway.
>
> That said, I don't understand why your live migrations are failing,
> especially not on the migration _to_ the modified Xen, since the idea
> you describe should be safe.
>
Could it be possible that xen is in log_dirty_mode when a DomU is
immigrating into it ?
> Cheers,
>
> Tim.
>
>> I just want to make the faulted page and its following
>> pages writable and marked as dirty after a write_protect
>> page_fault. The function "sh_prebuild" is called in the
>> sh_page_fault() right before sh_prefetch():
>>
>> ---------------sh_page_fault()-----------------
>>
>> if ( ( shadow_mode_log_dirty(v->domain) )
>> && ( ft == ft_demand_write ) )
>> sh_prebuild(.......);
>>
>> ..............
>>
>> I got problems when I migrate. The code can successfully
>> migrate the domainU to another UNMODIFIED Xen. But when I
>> migrate a domainU from UNMODIFIED Xen back to this MODIFIED
>> Xen, the domU's clock is frozen, I use "xm console" to get
>> in domU's console, but I just got about 6 TSC error messages
>> and no other respond.
>>
>> I am using 32-bit PV domain, no-PSE.
>> Does someone know any reasons? Thanks.
>>
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
>

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