Re: [Xen-devel] [PATCH 6/7] xen/setup: Make dom0_mem=XGB behavior be similar to classic Xen kernels.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Apr 03, 2012 at 09:58:44AM +0100, David Vrabel wrote:
> On 30/03/12 21:37, Konrad Rzeszutek Wilk wrote:
> > Meaning that we will allocate up to XGB and not consider the
> > rest of the memory as a possible balloon goal.
> 
> I agree with Jan when he commented on the equivalent Xen patch for this
> behaviour.  The current behaviour is better than the classic one.

Current behavior in the hypervisor or with current Linux kernel?
> 
> With your new behaviour it will no longer possible to specify an
> unlimited balloon but a limited number of initial pages.  This is
> behaviour that Jan said he used.

I am not sure I see the problem - I mean if one uses:

dom0_mem=min:8G,max:16G

I understand that we want to start at 8GB and if the user
choose to - balloon up to 16GB.

But doing this:

dom0_mem=8G

and allocating pagetables up to .. say 32GB, seems counter-intuive
as the effect is similar to having no 'dom0_mem' except that the initial
size is smaller.

> 
> This problem is better solved by improving the documentation.  A review
> of the xen.org wiki where dom0_mem is mentioned would be a good start,
> and an update to the recently added section for distro developers.
> 
> David
> 
> > This results in /proc/meminfo reporting:
> > 
> > -MemTotal:        2845024 kB
> > -MemFree:         2497716 kB
> > +MemTotal:        2927192 kB
> > +MemFree:         2458952 kB
> >  ...
> > -DirectMap4k:     8304640 kB
> > +DirectMap4k:     3063808 kB
> >  DirectMap2M:           0 kB
> > 
> > on a 8GB machine with 'dom0_mem=3GB' on the Xen hypervisor line.
> > 
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > ---
> >  arch/x86/xen/setup.c |   16 ++++++++++++++++
> >  1 files changed, 16 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> > index 2a12143..4e4aa8e 100644
> > --- a/arch/x86/xen/setup.c
> > +++ b/arch/x86/xen/setup.c
> > @@ -261,11 +261,27 @@ static unsigned long __init xen_get_max_pages(void)
> >  	 * the current maximum rather than the static maximum. In this
> >  	 * case the e820 map provided to us will cover the static
> >  	 * maximum region.
> > +	 *
> > +	 * The dom0_mem=min:X,max:Y tweaks options differently depending
> > +	 * on the version, but in general this is what we get:
> > +	 *                | XENMEM_maximum_reser  | nr_pages
> > +	 * --------------++-----------------------+-------------------
> > +	 *  no dom0_mem   | INT_MAX               | the max_phys_pfn
> > +	 *  =3G           | INT_MAX               | 786432
> > +	 *  =max:3G       | 786432                | 786432
> > +	 *  =min:1G,max:3G| 262144                | 786432
> > +	 *
> > +	 * The =3G is often used and it lead to us initially setting
> > +	 * 786432 and allowing dom0 to balloon up to the max_physical_pfn.
> > +	 * This is at odd with the classic XenOClassic so lets emulate
> > +	 * the classic behavior.
> >  	 */
> >  	if (xen_initial_domain()) {
> >  		ret = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
> >  		if (ret > 0)
> >  			max_pages = ret;
> > +		if (ret == -1UL)
> > +			max_pages = xen_start_info->nr_pages;
> >  	}
> >  
> >  	return min(max_pages, MAX_DOMAIN_PAGES);
> 
> 
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