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

[Jürgen Groß <jgross@xxxxxxxx>]

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.


Juergen

> +       if (current_pages <= 0) {
> +               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;
> +               }
>         }
>   
>   	balloon_stats.current_pages = current_pages;

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