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

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Message-ID: <596b08db-dc0e-4d42-ac17-570ca6e06bca@suse.com>
Date: Wed, 28 Jan 2026 12:31:13 +0100
From: Jürgen Groß <jgross@...e.com>
To: Roger Pau Monne <roger.pau@...rix.com>, xen-devel@...ts.xenproject.org,
 linux-kernel@...r.kernel.org
Cc: Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH 2/2] xen/balloon: improve accuracy of initial balloon
 target for dom0

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@...rix.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.


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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