virt: Lift the maximum RAM limit from 30GB to 255GB

The virt board restricts guests to only 30GB of RAM. This is a
hangover from the vexpress-a15 board, and there's no inherent reason
for it. 30GB is smaller than you might reasonably want to provision
a VM for on a beefy server machine. Raise the limit to 255GB.

We choose 255GB because the available space we currently have
below the 1TB boundary is up to the 512GB mark, but we don't
want to paint ourselves into a corner by assigning it all to
RAM. So we make half of it available for RAM, with the 256GB..512GB
range available for future non-RAM expansion purposes.

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

The last of these is obviously the trickiest, but 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.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Tested-by: Wei Huang <wei@redhat.com>
Message-id: 1456402182-11651-1-git-send-email-peter.maydell@linaro.org

1 files changed, 19 insertions, 2 deletions

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 44bbbea92b..7a56b4671b 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);
     }