@@ -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,
@@ -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.
+ /*
+ * 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 (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).
- */
- 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);
+ }
}
}
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. Signed-off-by: Gerd Hoffmann <kraxel@redhat.com> --- hw/i386/pc.c | 2 +- hw/i386/pc_piix.c | 61 +++++++++++++++++++++++++++++++++++-------------------- 2 files changed, 40 insertions(+), 23 deletions(-)