On 02/25/2016 06:09 AM, Peter Maydell wrote:
> The virt board restricts guests to only 30GB of RAM. This is a
> hangover from the vexpress-a15 board, and there's 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.mayd...@linaro.org>
> ---
> CC'ing kvm-arm as a heads-up that my proposal here is to make
> the kernel devs do the heavy lifting for supporting >255GB.
> Discussion welcome on whether I have the tradeoffs here right.
> ---
> hw/arm/virt.c | 21 +++++++++++++++++++--
> 1 file changed, 19 insertions(+), 2 deletions(-)
>
> 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)
I managed to test this patch with a 60GB guest VM on a 64GB machine,
which is the largest I can get hold of. I haven't seen any larger
machine, but guess it should be fine for VM to have >60GB memory by
reading the code.
Tested-by: Wei Huang <w...@redhat.com>
> +
> /* 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);
> }
>
>
