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

[Gerd Hoffmann <kraxel@redhat.com>]

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On Do, 2016-06-23 at 17:18 +0100, Anthony PERARD wrote:
> On Thu, Jun 23, 2016 at 04:57:54PM +0200, Gerd Hoffmann wrote:
> >   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. Unfortunately, it does not work :(.
> 
> In this patch, max_ram_below_4g is set before the call to xen_ram_init()
> and xen_ram_init read it back (via object_property_get_int()).  So, in
> xen_ram_init, user_lowmem is not 0.

Ah, I see.  We do the split calculation twice on xen.  That is pretty
pointless.  New patch attached.

cheers,
  Gerd


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From a1bb0d4f7a94e97102e7ea72d0a65de2a17b1160 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).

Also skip the whole ram split calculation on Xen.  xen_ram_init()
does its own split calculation anyway so it is superfluous, also
this way xen_ram_init can actually see whenever max-ram-below-4g
is zero or not.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/i386/pc.c      |  2 +-
 hw/i386/pc_piix.c | 52 +++++++++++++++++++++++++++++-----------------------
 hw/i386/pc_q35.c  |  3 +++
 xen-hvm.c         |  3 +++
 4 files changed, 36 insertions(+), 24 deletions(-)

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..f51fa77 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -108,37 +108,43 @@ static void pc_init1(MachineState *machine,
      *    so legacy non-PAE guests can get as much memory as possible in
      *    the 32bit address space below 4G.
      *
+     *  - Note that Xen has its own ram setp code in xen_ram_init(),
+     *    called via xen_hvm_init().
+     *
      * Examples:
      *    qemu -M pc-1.7 -m 4G    (old default)    -> 3584M low,  512M high
      *    qemu -M pc -m 4G        (new default)    -> 3072M low, 1024M high
      *    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)
      */
-    lowmem = pcms->max_ram_below_4g;
-    if (machine->ram_size >= pcms->max_ram_below_4g) {
-        if (pcmc->gigabyte_align) {
-            if (lowmem > 0xc0000000) {
-                lowmem = 0xc0000000;
-            }
-            if (lowmem & ((1ULL << 30) - 1)) {
-                error_report("Warning: Large machine and max_ram_below_4g "
-                             "(%" PRIu64 ") not a multiple of 1G; "
-                             "possible bad performance.",
-                             pcms->max_ram_below_4g);
-            }
-        }
-    }
-
-    if (machine->ram_size >= lowmem) {
-        pcms->above_4g_mem_size = machine->ram_size - lowmem;
-        pcms->below_4g_mem_size = lowmem;
-    } else {
-        pcms->above_4g_mem_size = 0;
-        pcms->below_4g_mem_size = machine->ram_size;
-    }
-
     if (xen_enabled()) {
         xen_hvm_init(pcms, &ram_memory);
+    } else {
+        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) {
+                if (lowmem > 0xc0000000) {
+                    lowmem = 0xc0000000;
+                }
+                if (lowmem & ((1ULL << 30) - 1)) {
+                    error_report("Warning: Large machine and max_ram_below_4g "
+                                 "(%" PRIu64 ") not a multiple of 1G; "
+                                 "possible bad performance.",
+                                 pcms->max_ram_below_4g);
+                }
+            }
+        }
+
+        if (machine->ram_size >= lowmem) {
+            pcms->above_4g_mem_size = machine->ram_size - lowmem;
+            pcms->below_4g_mem_size = lowmem;
+        } else {
+            pcms->above_4g_mem_size = 0;
+            pcms->below_4g_mem_size = machine->ram_size;
+        }
     }
 
     pc_cpus_init(pcms);
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