Re: [Qemu-devel] Change of max-ram-below-4g initial value breaks Xen

[Gerd Hoffmann <kraxel@redhat.com>]

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  Hi,

> How could xen_ram_init() find out if the value of max-ram-below-4g is
> the default or if a user have set it? Is there another way we could fix
> this?

Attached patch should fix it.  Patch survived a quick smoke test on kvm
so far, need to do some more testing tomorrow.  Can you give it a spin
on xen?

thanks,
  Gerd


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From d45a95861def18a02e1c26d3717693432517107a Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Thu, 23 Jun 2016 16:49:03 +0200
Subject: [PATCH] xen: fix ram init regression

Commit "8156d48 pc: allow raising low memory via max-ram-below-4g
option" causes a regression on xen, because it uses a different
memory split.

This patch initializes max-ram-below-4g to zero and leaves the
initialization to the memory initialization functions.  That way
they can pick different default values (max-ram-below-4g is zero
still) or use the user supplied value (max-ram-below-4g is non-zero).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/i386/pc.c      | 2 +-
 hw/i386/pc_piix.c | 3 +++
 hw/i386/pc_q35.c  | 3 +++
 xen-hvm.c         | 3 +++
 4 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 7198ed5..66e1dae 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1886,7 +1886,7 @@ static void pc_machine_initfn(Object *obj)
                         pc_machine_get_hotplug_memory_region_size,
                         NULL, NULL, NULL, &error_abort);
 
-    pcms->max_ram_below_4g = 0xe0000000; /* 3.5G */
+    pcms->max_ram_below_4g = 0; /* use default */
     object_property_add(obj, PC_MACHINE_MAX_RAM_BELOW_4G, "size",
                         pc_machine_get_max_ram_below_4g,
                         pc_machine_set_max_ram_below_4g,
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 53bc968..78e3d44 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -114,6 +114,9 @@ static void pc_init1(MachineState *machine,
      *    qemu -M pc,max-ram-below-4g=2G -m 4G     -> 2048M low, 2048M high
      *    qemu -M pc,max-ram-below-4g=4G -m 3968M  -> 3968M low (=4G-128M)
      */
+    if (!pcms->max_ram_below_4g) {
+        pcms->max_ram_below_4g = 0xe0000000; /* default: 3.5G */
+    }
     lowmem = pcms->max_ram_below_4g;
     if (machine->ram_size >= pcms->max_ram_below_4g) {
         if (pcmc->gigabyte_align) {
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index e4b541f..1b653e2 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -93,6 +93,9 @@ static void pc_q35_init(MachineState *machine)
     /* Handle the machine opt max-ram-below-4g.  It is basically doing
      * min(qemu limit, user limit).
      */
+    if (!pcms->max_ram_below_4g) {
+        pcms->max_ram_below_4g = 1ULL << 32; /* default: 4G */;
+    }
     if (lowmem > pcms->max_ram_below_4g) {
         lowmem = pcms->max_ram_below_4g;
         if (machine->ram_size - lowmem > lowmem &&
diff --git a/xen-hvm.c b/xen-hvm.c
index 98ea44f..eb57792 100644
--- a/xen-hvm.c
+++ b/xen-hvm.c
@@ -190,6 +190,9 @@ static void xen_ram_init(PCMachineState *pcms,
     /* Handle the machine opt max-ram-below-4g.  It is basically doing
      * min(xen limit, user limit).
      */
+    if (!user_lowmem) {
+        user_lowmem = HVM_BELOW_4G_RAM_END; /* default */
+    }
     if (HVM_BELOW_4G_RAM_END <= user_lowmem) {
         user_lowmem = HVM_BELOW_4G_RAM_END;
     }
-- 
1.8.3.1