[Xen-changelog] [linux-2.6.18-xen] xen/balloon: Fix return value interpretation for XENMEM_get_pod_target

["Xen patchbot-linux-2.6.18-xen" <patchbot-linux-2.6.18-xen@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1269970114 -3600
# Node ID 11175e60d39349f7daf5672481f25b9546f0918e
# Parent  10eae161c15390fdf31cd210b36540d608c46801
xen/balloon: Fix return value interpretation for XENMEM_get_pod_target

Unfortunately c/s 989 didn't consider what I would call a quirk in
pre-3.4 Xen, resulting in XENMEM_get_pod_target calls to not return
-ENOSYS as one would normally expect.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
---
 drivers/xen/balloon/balloon.c |   17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)

diff -r 10eae161c153 -r 11175e60d393 drivers/xen/balloon/balloon.c
--- a/drivers/xen/balloon/balloon.c     Thu Mar 25 10:00:03 2010 +0000
+++ b/drivers/xen/balloon/balloon.c     Tue Mar 30 18:28:34 2010 +0100
@@ -522,6 +522,7 @@ static int __init balloon_init(void)
        } xen_pod_target_t;
 # endif
        xen_pod_target_t pod_target = { .domid = DOMID_SELF };
+       int rc;
 #elif defined(CONFIG_X86)
        unsigned long pfn;
        struct page *page;
@@ -536,11 +537,17 @@ static int __init balloon_init(void)
        bs.current_pages = min(xen_start_info->nr_pages, max_pfn);
        totalram_pages   = bs.current_pages;
 #else 
-       totalram_bias = HYPERVISOR_memory_op(
-               HYPERVISOR_memory_op(XENMEM_get_pod_target,
-                       &pod_target) != -ENOSYS
-               ? XENMEM_maximum_reservation
-               : XENMEM_current_reservation,
+       rc = HYPERVISOR_memory_op(XENMEM_get_pod_target, &pod_target);
+       /*
+        * Xen prior to 3.4.0 masks the memory_op command to 4 bits, thus
+        * converting XENMEM_get_pod_target to XENMEM_decrease_reservation.
+        * Fortunately this results in a request with all input fields zero,
+        * but (due to the way bit 4 and upwards get interpreted) a starting
+        * extent of 1. When start_extent > nr_extents (>= in newer Xen), we
+        * simply get start_extent returned.
+        */
+       totalram_bias = HYPERVISOR_memory_op(rc != -ENOSYS && rc != 1
+               ? XENMEM_maximum_reservation : XENMEM_current_reservation,
                &pod_target.domid);
        if ((long)totalram_bias != -ENOSYS) {
                BUG_ON(totalram_bias < totalram_pages);

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