[PATCH 6/7] xen/setup: Make dom0_mem=XGB behavior be similar to classic Xen kernels.

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

Meaning that we will allocate up to XGB and not consider the
rest of the memory as a possible balloon goal.

This results in /proc/meminfo reporting:

-MemTotal:        2845024 kB
-MemFree:         2497716 kB
+MemTotal:        2927192 kB
+MemFree:         2458952 kB
 ...
-DirectMap4k:     8304640 kB
+DirectMap4k:     3063808 kB
 DirectMap2M:           0 kB

on a 8GB machine with 'dom0_mem=3GB' on the Xen hypervisor line.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/xen/setup.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 2a12143..4e4aa8e 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -261,11 +261,27 @@ static unsigned long __init xen_get_max_pages(void)
 	 * the current maximum rather than the static maximum. In this
 	 * case the e820 map provided to us will cover the static
 	 * maximum region.
+	 *
+	 * The dom0_mem=min:X,max:Y tweaks options differently depending
+	 * on the version, but in general this is what we get:
+	 *                | XENMEM_maximum_reser  | nr_pages
+	 * --------------++-----------------------+-------------------
+	 *  no dom0_mem   | INT_MAX               | the max_phys_pfn
+	 *  =3G           | INT_MAX               | 786432
+	 *  =max:3G       | 786432                | 786432
+	 *  =min:1G,max:3G| 262144                | 786432
+	 *
+	 * The =3G is often used and it lead to us initially setting
+	 * 786432 and allowing dom0 to balloon up to the max_physical_pfn.
+	 * This is at odd with the classic XenOClassic so lets emulate
+	 * the classic behavior.
 	 */
 	if (xen_initial_domain()) {
 		ret = HYPERVISOR_memory_op(XENMEM_maximum_reservation, &domid);
 		if (ret > 0)
 			max_pages = ret;
+		if (ret == -1UL)
+			max_pages = xen_start_info->nr_pages;
 	}
 
 	return min(max_pages, MAX_DOMAIN_PAGES);
-- 
1.7.7.5