Re: [PATCH v2 1/2] xen/balloon: Notify a host about a guest memory size limit

[Ian Campbell <Ian.Campbell@citrix.com>]

On Mon, 2013-04-29 at 12:37 +0100, Daniel Kiper wrote:
> Notify a host about a guest memory size limit.
> 
> Idea of this patch was discussed with Ian Campbell and Ian Jackson.
> 
> Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
> ---
>  drivers/xen/balloon.c |   25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index a56776d..856661f 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -65,6 +65,7 @@
>  #include <xen/balloon.h>
>  #include <xen/features.h>
>  #include <xen/page.h>
> +#include <xen/xenbus.h>
>  
>  /*
>   * balloon_process() state:
> @@ -586,6 +587,9 @@ static void __init balloon_add_region(unsigned long start_pfn,
>  static int __init balloon_init(void)
>  {
>  	int i;
> +#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
> +	int rc;
> +#endif
>  
>  	if (!xen_domain())
>  		return -ENODEV;
> @@ -621,6 +625,27 @@ static int __init balloon_init(void)
>  			balloon_add_region(PFN_UP(xen_extra_mem[i].start),
>  					   PFN_DOWN(xen_extra_mem[i].size));
>  
> +#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
> +	/*
> +	 * Notify a host about our memory size limit.
> +	 *
> +	 * The guest-max value is in KiB, so MAX_DOMAIN_PAGES should
> +	 * be converted respectively. PAGE_SHIFT converts pages to bytes,
> +	 * hence PAGE_SHIFT - 10.
> +	 */
> +	rc = xenbus_printf(XBT_NIL, "memory", "guest-max", "%lu",
> +				MAX_DOMAIN_PAGES << (PAGE_SHIFT - 10));
> +
> +	if (rc < 0)
> +		pr_info("xen/balloon: Memory hotplug may not be supported "
> +			"in some environments: %i\n", rc);
> +#else
> +	xenbus_printf(XBT_NIL, "memory", "guest-max", "%lu",

Would it be OK to not do anything in the non-hotplug case? That's just
the historical behaviour right?

Would you expect this value to differ from static-max?

> +			(balloon_stats.current_pages +
> +			 balloon_stats.balloon_low +
> +			 balloon_stats.balloon_high) << (PAGE_SHIFT - 10));
> +#endif
> +
>  	return 0;
>  }
>