Re: [Xen-devel] PoD, 4.2, and current/maximum reservation

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Feb 11, 2013 at 12:03 AM, James Harper <james.harper@xxxxxxxxxxxxxxxx> wrote:

A user has pointed out a problem with GPLPV under Xen 4.2 when using PoD. I'm using the difference between XENMEM_maximum_reservation and XENMEM_current_reservation to tell me how much I should balloon down to account for PoD, but when the user has ballooned down to 1G (from 4Gb or 8GB), GPLPV logs the following:

13005008825593: XenPCI     XENMEM_maximum_reservation = 262400
13005008825593: XenPCI     XENMEM_current_reservation = 262136
13005008825609: XenPCI     Trying to give 1056 KB (1 MB) to Xen

What is the correct way to tell how much PoD memory there is under 4.2? Am I doing it wrong?

I balloon down as early as possible (before xenbus starts) to avoid windows going over its limit so I'm hoping I can determine the size of PoD memory just via hypercalls.

You shouldn't have to know anything specifically about PoD -- you should just look at the balloon target for the guest written in xenstore.  The idea was as much as possible for the toolstack and Xen to work together to make it transparent to the balloon driver, in part because we expected to be running legacy drivers.  The Citrix PV drivers don't do anything special wrt PoD memory.  (Paul, please correct me if I'm wrong.)

WRT timing and xenbus, a couple of things:

* Windows does a scrub of all memory at start-of-day.  Especially on multiple-vcpu systems, we were unable to start the balloon process early enough to win the race against the scrubber, so we had to have ways of "reclaiming" zeroed pages for the PoD pool.  What this means is that it's not a matter of Windows *touching* memory, but of windows *dirtying* memory that will lead to a problem.

* So there is a minimum amount of memory Windows needs to be able to make it to the stage where the balloon driver can run.  When XenServer first implemented DMC, the team did extensive testing to determine minimum values above which Windows never crashed or hung, and (as I understand it) baked those into the xapi toolstack as a "seatbelt" to prevent users from setting the value too low.

Not sure if that helps in your particular case -- I think 1G was within the limit, but I could be wrong.  Dave, any comments?

 -George

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