Re: [PATCH 2/2] xen/balloon: improve accuracy of initial balloon target for dom0

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jan 28, 2026 at 12:31:13PM +0100, Jürgen Groß wrote:
> On 28.01.26 12:05, Roger Pau Monne wrote:
> > The dom0 balloon target set by the toolstack is the value returned by
> > XENMEM_current_reservation.  Do the same in the kernel balloon driver and
> > set the current allocation to the value returned by
> > XENMEM_current_reservation.  On my test system this causes the kernel
> > balloon driver target to exactly match the value set by the toolstack in
> > xenstore.
> > 
> > Note this approach can be used by both PV and PVH dom0s, as the toolstack
> > always uses XENMEM_current_reservation to set the initial target regardless
> > of the dom0 type.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> > ---
> >   drivers/xen/balloon.c | 27 +++++++++++++++++----------
> >   1 file changed, 17 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> > index 8c44a25a7d2b..9b6531eb28b6 100644
> > --- a/drivers/xen/balloon.c
> > +++ b/drivers/xen/balloon.c
> > @@ -724,7 +724,8 @@ static int __init balloon_add_regions(void)
> >   static int __init balloon_init(void)
> >   {
> >   	struct task_struct *task;
> > -	unsigned long current_pages;
> > +	long current_pages = 0;
> > +	domid_t domid = DOMID_SELF;
> >   	int rc;
> >   	if (!xen_domain())
> > @@ -732,15 +733,21 @@ static int __init balloon_init(void)
> >   	pr_info("Initialising balloon driver\n");
> > -	if (xen_pv_domain()) {
> > -		if (xen_released_pages >= xen_start_info->nr_pages)
> > -			goto underflow;
> > -		current_pages = min(xen_start_info->nr_pages -
> > -		                    xen_released_pages, max_pfn);
> > -	} else {
> > -		if (xen_unpopulated_pages >= get_num_physpages())
> > -			goto underflow;
> > -		current_pages = get_num_physpages() - xen_unpopulated_pages;
> > +	if (xen_initial_domain())
> > +		current_pages = HYPERVISOR_memory_op(XENMEM_current_reservation,
> > +		                                     &domid);
> 
> Is there any specific reason why this should be limited to dom0?
> 
> I _think_ this should work for other domains, too.

Sadly it doesn't, I've already tested.  The value returned by
XENMEM_current_reservation on PV guests is slightly different than
what's in xen_start_info->nr_pages, which exactly what the toolstack
writes in xenstore.  I assume there's some other stuff that's
accounted for in d->tot_pages, but don't really know what I'm afraid.

And in the HVM/PVH case using XENMEM_current_reservation for domUs
would also take into account the Video memory, which skews the target.

This is the best I could do I'm afraid, at the expense of having so
many different way to fetch the information.

Thanks, Roger.