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Re: [Xen-devel] [PATCH] Disallow setting maxmem to higher value than tot

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Disallow setting maxmem to higher value than total physical memory size
From: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Date: Wed, 1 Sep 2010 18:56:25 +0100
Cc: Michal Novotny <minovotn@xxxxxxxxxx>, "'xen-devel@xxxxxxxxxxxxxxxxxxx'" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Wed, 2010-09-01 at 17:53 +0100, Jeremy Fitzhardinge wrote:
> On 09/01/2010 05:44 AM, Ian Campbell wrote:
> > On Wed, 2010-09-01 at 13:31 +0100, Michal Novotny wrote:
> >> Hi,
> >> this is the patch to disallow changing the maxmem value to higher value 
> >> than total physical memory size since without this patch I was able to 
> >> set dom0 maxmem to higher (invalid) value which is not correct.
> > I think it is allowable for a domU though. Consider the scenario where
> > you have two hosts, one of which has more physical RAM than the other.
> > You may which to boot a domain on the smaller host, (i.e. booting
> > ballooned with a current_pages suitable for the small host) and then
> > migrate it to the large machine where you then want to be able to
> > balloon to a value larger than was even possible on the previous
> > machine.
> But max-mem can change between hosts; on the small host it needn't have
> a maxmem larger than the host's memory.  (The domain itself may have a
> larger notion of maxmem internally, but that's separate.)

It's not separate, this guest configuration item precisely informs the
guest how large it can expect it's memory map to ever need to be (the
setting is also called the static-max in both xend and xapi).

For a PV guest the value is pushed down into the hypervisor by the
toolstack via XENMEM_set_memory_map and this controls the memory map
returned to the guest from XENMEM_get_memory_map, which in turn informs
the guest's choice of maxmem value (for PV guests which can boot
ballooned that is).

For an HVM guest maxmem sets (via HVM loader) the limit of the e820
presented to the guest.


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