pc & q35: Add new machine opt max-ram-below-4g

This is a pc & q35 only machine opt.

If you add enough PCI devices then all mmio for them will not fit
below 4G which may not be the layout the user wanted. This allows
you to increase the below 4G address space that PCI devices can use
(aka decrease ram below 4G) and therefore in more cases not have any
mmio that is above 4G.

For example using "-machine pc,max-ram-below-4g=2G" on the command
line will limit the amount of ram that is below 4G to 2G.

Note: this machine option cannot be used to increase the amount
of ram below 4G.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MST: fix 32 bit

1 files changed, 21 insertions, 1 deletions

diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index da5fd53304..155db99f63 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -44,6 +44,7 @@
 #include "hw/ide/ahci.h"
 #include "hw/usb.h"
 #include "hw/cpu/icc_bus.h"
+#include "qemu/error-report.h"
 
 /* ICH9 AHCI has 6 ports */
 #define MAX_SATA_PORTS     6
@@ -85,6 +86,7 @@ static void pc_q35_init(MachineState *machine)
     PCIDevice *ahci;
     DeviceState *icc_bridge;
     PcGuestInfo *guest_info;
+    ram_addr_t lowmem;
 
     /* Check whether RAM fits below 4G (leaving 1/2 GByte for IO memory
      * and 256 Mbytes for PCI Express Enhanced Configuration Access Mapping
@@ -96,7 +98,25 @@ static void pc_q35_init(MachineState *machine)
      * breaking migration.
      */
     if (machine->ram_size >= 0xb0000000) {
-        ram_addr_t lowmem = gigabyte_align ? 0x80000000 : 0xb0000000;
+        lowmem = gigabyte_align ? 0x80000000 : 0xb0000000;
+    } else {
+        lowmem = 0xb0000000;
+    }
+
+    /* Handle the machine opt max-ram-below-4g.  It is basicly doing
+     * min(qemu limit, user limit).
+     */
+    if (lowmem > pc_machine->max_ram_below_4g) {
+        lowmem = pc_machine->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.",
+                         pc_machine->max_ram_below_4g);
+        }
+    }
+
+    if (machine->ram_size >= lowmem) {
         above_4g_mem_size = machine->ram_size - lowmem;
         below_4g_mem_size = lowmem;
     } else {