Re: [Xen-devel] Re: PoD issue

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

Yeah, the OSS tree doesn't get the kind of regression testing it
really needs at the moment.  I was using the OSS balloon drivers when
I implemented and submitted the PoD code last year.  I didn't have any
trouble then, and I was definitely using up all of the memory.  But I
haven't done any testing on OSS since then, basically.

 -George

On Thu, Feb 4, 2010 at 12:17 AM, Jan Beulich <JBeulich@xxxxxxxxxx> wrote:
> It was in the balloon driver's interaction with xenstore - see 2.6.18 c/s
> 989.
>
> I have to admit that I cannot see how this issue could slip attention
> when the PoD code was introduced - any guest with PoD in use and
> an unfixed balloon driver is set to crash sooner or later (implying the
> unfortunate effect of requiring an update of the pv drivers in HVM
> guests when upgrading Xen from a PoD-incapable to a PoD-capable
> version).
>
> Jan
>
>>>> George Dunlap <george.dunlap@xxxxxxxxxxxxx> 03.02.10 19:42 >>>
> So did you track down where the math error is?  Do we have a plan to fix
> this going forward?
>  -George
>
> Jan Beulich wrote:
>>>>> George Dunlap  01/29/10 7:30 PM >>>
>>>>>
>>> PoD is not critical to balloon out guest memory.  You can boot with mem
>>> == maxmem and then balloon down afterwards just as you could before,
>>> without involving PoD.  (Or at least, you should be able to; if you
>>> can't then it's a bug.)  It's just that with PoD you can do something
>>> you've always wanted to do but never knew it: boot with 1GiB with the
>>> option of expanding up to 2GiB later. :-)
>>>
>>
>> Oh, no, that's not what I meant. What I really wanted to say is that
>> with PoD, a properly functioning balloon driver in the guest is crucial
>> for it to stay alive long enough.
>>
>>
>>> With the 54 megabyte difference: It's not like a GiB vs GB thing, is
>>> it?  (i.e., 2^30 vs 10^9?)  The difference between 1GiB (2^30) and 1 GB
>>> (10^9) is about 74 megs, or 18,000 pages.
>>>
>>
>> No, that's not the problem. As I understand it now, the problem is
>> that totalram_pages (which the balloon driver bases its calculations
>> on) reflects all memory available after all bootmem allocations were
>> done (i.e. includes neither the static kernel image nor any memory
>> allocated before or from the bootmem allocator).
>>
>>
>>> I guess that is a weakness of PoD in general: we can't control the guest
>>> balloon driver, but we rely on it to have the same model of how to
>>> translate "target" into # pages in the balloon as the PoD code.
>>>
>>
>> I think this isn't a weakness of PoD, but a design issue in the balloon
>> driver's xenstore interface: While a target value shown in or obtained
>> from the /proc and /sys interfaces naturally can be based on (and
>> reflect) any internal kernel state, the xenstore interface should only
>> use numbers in terms of full memory amount given to the guest.
>> Hence a target value read from the memory/target node should be
>> adjusted before put in relation to totalram_pages. And I think this
>> is a general misconception in the current implementation (i.e. it
>> should be corrected not only for the HVM case, but for the pv one
>> as well).
>>
>> The bad aspect of this is that it will require a fixed balloon driver
>> in any HVM guest that has maxmem>mem when the underlying Xen
>> gets updated to a version that supports PoD. I cannot, however,
>> see an OS and OS-version independent alternative (i.e. something
>> to be done in the PoD code or the tools).
>>
>> Jan
>>
>>
>
>
>
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