Re: "practical" memory allocation limit?

[Laszlo Ersek <lersek@redhat.com>]

On 08/28/17 16:25, Shi, Steven wrote:
> Hi Laszlo,
> 
> I happen to have a question about how to correctly get the system memory size on Qemu.
> 
> In the QemuInitializeRam() of OvmfPkg\PlatformPei\MemDetect.c, I add debug info as below to trace the GetSystemMemorySizeBelow4gb() and GetSystemMemorySizeAbove4gb() output. But the output results seems not right if my input memory size > 4GB. See my commands and trace outputs in below.
> 
> 
> 
> MemDetect.c  Line 602:
> 
>   //
> 
>   // Determine total memory size available
> 
>   //
> 
>   LowerMemorySize = GetSystemMemorySizeBelow4gb ();
> 
>   UpperMemorySize = GetSystemMemorySizeAbove4gb ();
> 
>   DEBUG ((EFI_D_INFO, "LowerMemorySize= 0x%x\n", LowerMemorySize));
> 
>   DEBUG ((EFI_D_INFO, "UpperMemorySize= 0x%x\n", UpperMemorySize));
> 
> 
> 
> My test commands and trace outputs:
> 
> $ /opt/qemu/bin/qemu-system-x86_64 -m 5120 -enable-kvm -hda /home/jshi19/workspace/simics5-project/images/luv-v2.1_diskboot_gpt_x86_64_.img -machine pc-q35-2.9 -bios OVMF.fd -serial file:serial.log
> 
> LowerMemorySize= 0x80000000 //2GB, but should not it be 4GB?
> 
> UpperMemorySize= 0xC0000000 //3GB, but should not it be 1GB?

No, this is correct; the values returned are system memory *amounts*.
(On the QEMU command line, the "-m" switch says how much system memory
you have, not the highest address in system memory.)

In the 4GB address space, you don't have *just* system memory, you have
a whole bunch of MMIO too. (Most of that is used for 32-bit MMIO PCI BAR
allocation, some is used for the pflash chip range, LAPICs, etc.) On Q35
you also have 256MB assigned to the MMCONFIG / ECAM area (which provides
direct MMIO access to PCI config space for PCI 256 buses).

The physical memory map also depends on the board model; on i440fx, the
32-bit system memory can go up to 3GB, while on Q35, it only goes up to 2GB.

So, if you pass 5GB to a Q35 machine, 2GB of that are placed in the
address space at [0, 2G), and the other 3GB are placed in the address
space at [4G, 7G).

> $ /opt/qemu/bin/qemu-system-x86_64 -m 6144 -enable-kvm -hda /home/jshi19/workspace/simics5-project/images/luv-v2.1_diskboot_gpt_x86_64_.img -machine pc-q35-2.9 -bios OVMF.fd -serial file:serial.log
> 
> LowerMemorySize= 0x80000000 //2GB, but should not it be 4GB?
> 
> UpperMemorySize= 0x0                 // 0GB, but should not it be 2GB?

With 6G DRAM, 2G go low, and 4G go high.

(Your output is misleading because you used the %x format specifier in
your debug message -- %Lx would be correct, for printing a UINT64 value.)

Laszlo