Re: [PATCHv1] xen/balloon: disable memory hotplug in PV guests

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 03/09/2015 10:10 AM, David Vrabel wrote:
> Memory hotplug doesn't work with PV guests because:
>
>    a) The p2m cannot be expanded to cover the new sections.
>
>    b) add_memory() builds page tables for the new sections which means
>       the new pages must have valid p2m entries (or a BUG occurs).

Is this due to recent p2m rework?

-boris


>
> So, in PV guests, clamp the target to the current (== maximum) number
> of pages and do not try to hotplug any memory.
>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
> Cc: Daniel Kiper <daniel.kiper@oracle.com>
> ---
>   drivers/xen/balloon.c |   13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 0b52d92..4158196 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -225,6 +225,19 @@ static enum bp_state reserve_additional_memory(long credit)
>   	u64 hotplug_start_paddr;
>   	unsigned long balloon_hotplug = credit;
>   
> +	/*
> +	 * Memory hotplug doesn't work with PV guests because:
> +	 *
> +	 *   a) The p2m cannot be expanded to cover the new sections.
> +	 *
> +	 *   b) add_memory() builds page tables for the new sections
> +	 *      which means they must be fully populated in advance.
> +	 */
> +	if (xen_pv_domain()) {
> +		balloon_stats.target_pages = balloon_stats.current_pages;
> +		return BP_DONE;
> +	}
> +
>   	hotplug_start_paddr = PFN_PHYS(SECTION_ALIGN_UP(max_pfn));
>   	balloon_hotplug = round_up(balloon_hotplug, PAGES_PER_SECTION);
>   	nid = memory_add_physaddr_to_nid(hotplug_start_paddr);