Re: [PATCH] Partial revert "x86/xen: fix balloon target initialization for PVH dom0"

[James Dingwall <james@dingwall.me.uk>]

On Tue, Jan 20, 2026 at 03:06:47PM +0100, Roger Pau Monne wrote:
> This partially reverts commit 87af633689ce16ddb166c80f32b120e50b1295de so
> the current memory target for PV guests is still fetched from
> start_info->nr_pages, which matches exactly what the toolstack sets the
> initial memory target to.
> 
> Using get_num_physpages() is possible on PV also, but needs adjusting to
> take into account the ISA hole and the PFN at 0 not considered usable
> memory depite being populated, and hence would need extra adjustments.
> Instead of carrying those extra adjustments switch back to the previous
> code.  That leaves Linux with a difference in how current memory target is
> obtained for HVM vs PV, but that's better than adding extra logic just for
> PV.
> 
> Also, for HVM the target is not (and has never been) accurately calculated,
> as in that case part of what starts as guest memory is reused by hvmloader
> and possibly other firmware to store ACPI tables and similar firmware
> information, thus the memory is no longer being reported as RAM in the
> memory map.
> 
> Reported-by: James Dingwall <james@dingwall.me.uk>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
>  drivers/xen/balloon.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 49c3f9926394..e799650f6c8c 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -724,6 +724,7 @@ static int __init balloon_add_regions(void)
>  static int __init balloon_init(void)
>  {
>  	struct task_struct *task;
> +	unsigned long current_pages;
>  	int rc;
>  
>  	if (!xen_domain())
> @@ -731,12 +732,15 @@ static int __init balloon_init(void)
>  
>  	pr_info("Initialising balloon driver\n");
>  
> -	if (xen_released_pages >= get_num_physpages()) {
> +	current_pages = xen_pv_domain() ? min(xen_start_info->nr_pages, max_pfn)
> +	                                : get_num_physpages();
> +
> +	if (xen_released_pages >= current_pages) {
>  		WARN(1, "Released pages underflow current target");
>  		return -ERANGE;
>  	}
>  
> -	balloon_stats.current_pages = get_num_physpages() - xen_released_pages;
> +	balloon_stats.current_pages = current_pages - xen_released_pages;
>  	balloon_stats.target_pages  = balloon_stats.current_pages;
>  	balloon_stats.balloon_low   = 0;
>  	balloon_stats.balloon_high  = 0;
> -- 
> 2.51.0
> 

Thank you Roger, I tested this patch on the system which originally showed
the error and the pci passthrough now works as expected.

Regards,
James