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

["Jürgen Groß" <jgross@suse.com>]

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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@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.

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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