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RE: [Xen-devel] RE: Ballooning up

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel] RE: Ballooning up
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 13 Sep 2010 17:22:20 -0700 (PDT)
Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Xen-devel@xxxxxxxxxxxxxxxxxxx, Daniel Kiper <dkiper@xxxxxxxxxxxx>, Konrad Wilk <konrad.wilk@xxxxxxxxxx>
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> > I would call this dom0 functionality a bug.  I think both Citrix
> > and Oracle use dom0_mem as a normal boot option for every
> > installation and, while I think both employ heuristics to choose
> > a larger dom0_mem for larger physical memory, I don't think it
> > grows large enough for, say, >256GB physical memory, to accommodate
> > the necessarily large number of page tables.
> >
> > So, I'd vote for NOT allowing dom0 to balloon up to physical
> > memory if dom0_mem is specified, and possibly a kernel command
> > line option that allows it to grow beyond.  Or, possibly, no
> > option and never allow dom0 memory to grow beyond dom0_mem
> > unless (possibly) it grows with hot-plug.
> Yes, its a bit of a problem.  The trouble is that the kernel can't
> really distinguish the two cases; either way, it sees a Xen-supplied
> xen_start_info->nr_pages as the amount of initial memory available, and
> an E820 table referring to more RAM beyond that.
> I guess there are three options:
>    1. add a "xen_maxmem" (or something) kernel parameter to override
>       space specified in the E820 table
>    2. ignore E820 if its a privileged domain
>    3. only allow extra memory up to a certain ratio of the base memory
>       (8x? 16x? 32x?)
> I think the third is probably the simplest and least hacky, as it
> directly addresses the underlying issue (and prevents domU mishaps as
> well).

I like 2': ignore E820 if it is dom0 and dom0_mem has been specified.
This most closely conforms to current behavior in shipping systems
and I don't really see a use model for allowing dom0 memory to
grow beyond dom0_mem (if dom0_mem is specified).

(1) will most likely result in vendors specifying dom0_mem AND
    xen_maxmem to the same value, so IMHO will just be confusing
(2) for non-dom0 privileged domains, I can't offhand come up with
    a scenario where memory<>maxmem would be valuable, so this
    would be my second choice (after 2').
(3) seems like policy enforcement with insufficient information
    as the "correct" ratio might change in future kernels and
    we don't even know what it should be now (and it may be
    very kernel dependent?)


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