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

[Jürgen Groß <jgross@xxxxxxxx>]

On 28.01.26 12:54, Roger Pau Monné wrote:
> 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@xxxxxxxxxx>
> > > ---
> > >    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.
Meh, too bad.

For now I think we need to live with that. But for the future I'd like to
suggest to add a new Xenstore node memory/reservation-diff containing the
difference between XENMEM_current_reservation output and the tools' view
of the domain's memory size.

The balloon driver could read that node in balloon_init() and use
XENMEM_current_reservation in case the node is found.


Juergen

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