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

[Miroslav Rezanina <mrezanin@xxxxxxxxxx>]

----- Original Message -----
From: "Gianluca Guida" <gianluca.guida@xxxxxxxxxx>
To: "Miroslav Rezanina" <mrezanin@xxxxxxxxxx>
Cc: jeremy@xxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
Sent: Thursday, August 20, 2009 11:31:34 AM GMT +01:00 Amsterdam / Berlin / 
Bern / Rome / Stockholm / Vienna
Subject: [Xen-devel] [PATCH][v2.6.29][XEN] Return unused memory to hypervisor

Miroslav Rezanina writes:
> > 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.
> > 
>
> A good idea would be to avoid putting this code in the generic kernel
> code. For now just placing it in at the end of Xen's post-allocator
> init would make it completely transparent to the non-xen kernel.
>

Hi Gianluca,
good point. You're right. I moved calling to xen_post_allocator_init function. 
It is better place for it.

Regards,
Mirek

----diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index b58e963..2a9cc80 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -28,6 +28,7 @@
 #include <linux/console.h>
 
 #include <xen/interface/xen.h>
+#include <xen/interface/memory.h>
 #include <xen/interface/version.h>
 #include <xen/interface/physdev.h>
 #include <xen/interface/vcpu.h>
@@ -122,6 +123,36 @@ static int have_vcpu_info_placement =
 #endif
        ;
 
+/**
+ *  * 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);
+       }
+}
+
 
 static void xen_vcpu_setup(int cpu)
 {
@@ -1057,6 +1088,8 @@ static __init void xen_post_allocator_init(void)
        SetPagePinned(virt_to_page(level3_user_vsyscall));
 #endif
        xen_mark_init_mm_pinned();
+
+       xen_return_unused_mem();
 }
 
 /* This is called once we have the cpu_possible_map */

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