Re: [PATCH 1/5] xen: use maximum reservation to limit amount of usable RAM

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>  arch/x86/xen/setup.c |   19 +++++++++++++++++++
>  1 files changed, 19 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index df118a8..c3b8d44 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -184,6 +184,19 @@ static unsigned long __init xen_set_identity(const struct e820entry *list,
>  					PFN_UP(start_pci), PFN_DOWN(last));
>  	return identity;
>  }
> +
> +static unsigned long __init xen_get_max_pages(void)
> +{
> +	unsigned long max_pages = MAX_DOMAIN_PAGES;
> +	domid_t domid = DOMID_SELF;
> +	int ret;
> +
> +	ret = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
> +	if (ret > 0)
> +		max_pages = ret;
> +	return min(max_pages, MAX_DOMAIN_PAGES);
> +}
> +
>  /**
>   * machine_specific_memory_setup - Hook for machine specific memory setup.
>   **/
> @@ -292,6 +305,12 @@ char * __init xen_memory_setup(void)
>  
>  	sanitize_e820_map(e820.map, ARRAY_SIZE(e820.map), &e820.nr_map);
>  
> +	extra_limit = xen_get_max_pages();
> +	if (extra_limit >= max_pfn)
> +		extra_pages = extra_limit - max_pfn;
> +	else
> +		extra_pages = 0;
> +
>  	extra_pages += xen_return_unused_memory(xen_start_info->nr_pages, &e820);

I ran this on three setups:

1) PV (domU)
2) PV+PCI (dom0)
3) PV+PCI+e820_hole=1 (domU)

and then the same without this patch.

Both the 2) and 3) worked correctly - the E820 had the same non-RAM regions and
gaps - and the last RAM E820 entry was properly truncated. However, when it
came to pure PV it was truncated more than it should:

domU:								domU:
0000000000000000 - 00000000000a0000 (usable)			0000000000000000 - 00000000000a0000 (usable)
00000000000a0000 - 0000000000100000 (reserved)			00000000000a0000 - 0000000000100000 (reserved)
0000000000100000 - 0000000040800000 (usable)		      |	0000000000100000 - 0000000040100000 (usable)

(left has the old PV - without your patch). Which makes me think that there is something
amiss in the toolstack? I used 'xl' (latest xen-unstable from today).