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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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@xxxxxxxxxx>
> ---
> Cc: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> ---
>   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);

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