http://patchwork.ozlabs.org/patch/588094/

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 44bbbea..7a56b46 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -95,6 +95,23 @@ typedef struct {
 #define VIRT_MACHINE_CLASS(klass) \
     OBJECT_CLASS_CHECK(VirtMachineClass, klass, TYPE_VIRT_MACHINE)
 
+/* RAM limit in GB. Since VIRT_MEM starts at the 1GB mark, this means
+ * RAM can go up to the 256GB mark, leaving 256GB of the physical
+ * address space unallocated and free for future use between 256G and 512G.
+ * If we need to provide more RAM to VMs in the future then we need to:
+ *  * allocate a second bank of RAM starting at 2TB and working up
+ *  * fix the DT and ACPI table generation code in QEMU to correctly
+ *    report two split lumps of RAM to the guest
+ *  * fix KVM in the host kernel to allow guests with >40 bit address spaces
+ * (We don't want to fill all the way up to 512GB with RAM because
+ * we might want it for non-RAM purposes later. Conversely it seems
+ * reasonable to assume that anybody configuring a VM with a quarter
+ * of a terabyte of RAM will be doing it on a host with more than a
+ * terabyte of physical address space.)
+ */
+#define RAMLIMIT_GB 255
+#define RAMLIMIT_BYTES (RAMLIMIT_GB * 1024ULL * 1024 * 1024)
+
 /* Addresses and sizes of our components.
  * 0..128MB is space for a flash device so we can run bootrom code such as UEFI.
  * 128MB..256MB is used for miscellaneous device I/O.
@@ -130,7 +147,7 @@ static const MemMapEntry a15memmap[] = {
     [VIRT_PCIE_MMIO] =          { 0x10000000, 0x2eff0000 },
     [VIRT_PCIE_PIO] =           { 0x3eff0000, 0x00010000 },
     [VIRT_PCIE_ECAM] =          { 0x3f000000, 0x01000000 },
-    [VIRT_MEM] =                { 0x40000000, 30ULL * 1024 * 1024 * 1024 },
+    [VIRT_MEM] =                { 0x40000000, RAMLIMIT_BYTES },
     /* Second PCIe window, 512GB wide at the 512GB boundary */
     [VIRT_PCIE_MMIO_HIGH] =   { 0x8000000000ULL, 0x8000000000ULL },
 };
@@ -1066,7 +1083,7 @@ static void machvirt_init(MachineState *machine)
     vbi->smp_cpus = smp_cpus;
 
     if (machine->ram_size > vbi->memmap[VIRT_MEM].size) {
-        error_report("mach-virt: cannot model more than 30GB RAM");
+        error_report("mach-virt: cannot model more than %dGB RAM", RAMLIMIT_GB);
         exit(1);
     }