pc: allow raising low memory via max-ram-below-4g option

This patch extends the functionality of the max-ram-below-4g option
to also allow increasing lowmem.  Use case: Give as much memory as
possible to legacy non-PAE guests.

While being at it also rework the lowmem calculation logic and add a
longish comment describing how it works and what the compatibility
constrains are.

Note:  This is a incompatible change.  When setting max-ram-below-4g to
a value larger than 3.5G (or 3G with gigabyte alignment) it has no
effect on older qemu versions: qemu silently ignores it.  With the patch
applied it actually has an effect and changes the ram layout.  Highly
unlikely to hit in practive though as there is no reason start old qemu
versions that way.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1464857305-26675-1-git-send-email-kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 40 insertions, 23 deletions

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index e29ccc8341..bfaf5a399b 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1893,7 +1893,7 @@ static void pc_machine_initfn(Object *obj)
                         pc_machine_get_hotplug_memory_region_size,
                         NULL, NULL, NULL, &error_abort);
 
-    pcms->max_ram_below_4g = 1ULL << 32; /* 4G */
+    pcms->max_ram_below_4g = 0xe0000000; /* 3.5G */
     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 24e7042680..7b4790f86f 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -87,29 +87,46 @@ static void pc_init1(MachineState *machine,
     MemoryRegion *rom_memory;
     ram_addr_t lowmem;
 
-    /* Check whether RAM fits below 4G (leaving 1/2 GByte for IO memory).
-     * If it doesn't, we need to split it in chunks below and above 4G.
-     * In any case, try to make sure that guest addresses aligned at
-     * 1G boundaries get mapped to host addresses aligned at 1G boundaries.
-     * For old machine types, use whatever split we used historically to avoid
-     * breaking migration.
-     */
-    if (machine->ram_size >= 0xe0000000) {
-        lowmem = pcmc->gigabyte_align ? 0xc0000000 : 0xe0000000;
-    } else {
-        lowmem = 0xe0000000;
-    }
-
-    /* Handle the machine opt max-ram-below-4g.  It is basically doing
-     * min(qemu limit, user limit).
+    /*
+     * Calculate ram split, for memory below and above 4G.  It's a bit
+     * complicated for backward compatibility reasons ...
+     *
+     *  - Traditional split is 3.5G (lowmem = 0xe0000000).  This is the
+     *    default value for max_ram_below_4g now.
+     *
+     *  - Then, to gigabyte align the memory, we move the split to 3G
+     *    (lowmem = 0xc0000000).  But only in case we have to split in
+     *    the first place, i.e. ram_size is larger than (traditional)
+     *    lowmem.  And for new machine types (gigabyte_align = true)
+     *    only, for live migration compatibility reasons.
+     *
+     *  - Next the max-ram-below-4g option was added, which allowed to
+     *    reduce lowmem to a smaller value, to allow a larger PCI I/O
+     *    window below 4G.  qemu doesn't enforce gigabyte alignment here,
+     *    but prints a warning.
+     *
+     *  - Finally max-ram-below-4g got updated to also allow raising lowmem,
+     *    so legacy non-PAE guests can get as much memory as possible in
+     *    the 32bit address space below 4G.
+     *
+     * 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)
      */
-    if (lowmem > pcms->max_ram_below_4g) {
-        lowmem = pcms->max_ram_below_4g;
-        if (machine->ram_size - lowmem > lowmem &&
-            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);
+    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);
+            }
         }
     }