Re: [Xen-devel] [PATCH] x86: expose XENMEM_get_pod_target to subject domain

[George Dunlap <dunlapg@xxxxxxxxx>]

On Mon, Feb 24, 2014 at 1:31 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> Not having got any satisfactory suggestions on the inquiry on how to
> determine the amount a PoD guest needs to balloon down by (see
> http://lists.xenproject.org/archives/html/xen-devel/2014-01/msg01524.html
> and the thread following it), expose XENMEM_get_pod_target such that
> the guest can use it for this purpose.

So in theory the bug you're seeing has nothing to do with PoD -- it
just has to do with a different interpretation that the balloon driver
and Xen may have as to what "target" means.  Is that right?  The only
difference is that in the PoD case, not ballooning down enough can be
deadly to the domain; whereas in the non-PoD case, the worst that can
happen is that the toolstack has less memory left over on the host
than it may have expected.

I don't like the idea of exposing specific PoD information to the
guest -- PoD should be completely transparent to the guest.  If we
make it PoD-specific, we may end up with a different sized domain
depending on whether we booted with PoD mode or not.

Is the real problem that there's no way to determine the number of
potentially non-empty pfn ranges?  If we exposed the number of
non-empty p2m ranges (either ram or PoD), then the guest could compare
that to the target and balloon down as necessary, no?

> Also leverage some cleanup potential resulting from this change.

Cleanup should generally be done in separate patches, so that one
change can be reviewed at a time.

 -George

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