[Xen-devel] [PATCH][v2.6.29][XEN] Return unused memory to hypervisor

[Miroslav Rezanina <mrezanin@xxxxxxxxxx>]

Hi,

when running linux as XEN guest and use boot parameter mem= to set memory lower 
then is assigned to guest, not used memory should be returned to hypervisor as 
free. This is working with kernel available on xen.org pages, but is not 
working with kernel 2.6.29. Comparing both kernels I found code for returning 
unused memory to hypervisor is missing. Following patch add this functionality 
to 2.6.29 kernel.

Miroslav Rezanina <mrezanin@xxxxxxxxxx>
--
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 6a8811a..fd6b0e7 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -118,6 +118,10 @@ struct boot_params __initdata boot_params;
 struct boot_params boot_params;
 #endif
 
+#ifdef CONFIG_XEN
+void __init xen_return_unused_mem(void);
+#endif
+
 /*
  * Machine setup..
  */
@@ -920,6 +924,9 @@ void __init setup_arch(char **cmdline_p)
        paging_init();
        paravirt_pagetable_setup_done(swapper_pg_dir);
        paravirt_post_allocator_init();
+#ifdef CONFIG_XEN
+       xen_return_unused_mem();
+#endif
 
 #ifdef CONFIG_X86_64
        map_vsyscall();
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 15c6c68..bc5d2bc 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -20,6 +20,7 @@
 #include <xen/page.h>
 #include <xen/interface/callback.h>
 #include <xen/interface/physdev.h>
+#include <xen/interface/memory.h>
 #include <xen/features.h>
 
 #include "xen-ops.h"
@@ -34,6 +35,36 @@ extern void xen_syscall32_target(void);
 
 
 /**
+ * Author: Miroslav Rezanina <mrezanin@xxxxxxxxxx>
+ * Function retuns unused memory to hypevisor
+ **/
+void __init xen_return_unused_mem(void)
+{
+       if (xen_start_info->nr_pages > max_pfn) {
+        /*
+       * the max_pfn was shrunk (probably by mem=
+       * kernel parameter); shrink reservation with the HV
+       */
+       struct xen_memory_reservation reservation = {
+       .address_bits = 0,
+       .extent_order = 0,
+       .domid = DOMID_SELF
+       };
+       unsigned int difference;
+       int ret;
+
+       difference = xen_start_info->nr_pages - max_pfn;
+
+       set_xen_guest_handle(reservation.extent_start,
+               ((unsigned long *)xen_start_info->mfn_list) + max_pfn);
+       reservation.nr_extents = difference;
+       ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation,
+               &reservation);
+       BUG_ON (ret != difference);
+       }
+}
+
+/**
  * machine_specific_memory_setup - Hook for machine specific memory setup.
  **/
 

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