Re: [Qemu-devel] [PATCH] virt: Lift the maximum RAM limit from 30GB to 255GB

[Christoffer Dall <christoffer.dall@linaro.org>]

On Thu, Feb 25, 2016 at 04:51:51PM +0000, Peter Maydell wrote:
> [Typoed the kvmarm list address; sorry... -- PMM]
> 
> On 25 February 2016 at 12:09, Peter Maydell <peter.maydell@linaro.org> 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

did you mean "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>
> > ---
> > 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.

I think so, this looks good to me.

> > ---
> >  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)
> > +
> >  /* 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);
> >      }
> >
> > --
> > 1.9.1

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>