P-SeriesのトピックRe: last 256MB of the linux 32bit addresses space on p4080ds arch

last 256MB of the linux 32bit addresses space on p4080ds arch

yoav_rubin2 — Mon, 28 Oct 2019 12:23:14 GMT

I'm using 32bit linux 4.1.8 with Yocto environment on the P4080DS SoC and noticed that the last 256MB of the 32bit addresses space are reserved for some purpose.

from arch/powerpc/mm/init_32.c:

/*The amount of lowmem must be within 0xF0000000 - KERNELBASE. */

# if (CONFIG_LOWMEM_SIZE > (0xF0000000 - PAGE_OFFSET))

# error "You must adjust CONFIG_LOWMEM_SIZE or CONFIG_KERNEL_START"

# endif

I wanted to use addresses 0XF0000000 - 0XFFFFFFFF for lowmemory but I can't.

What are they reserved for? maybe vmalloc area? and is it possible to resize this reserved area (shrink it)?

thanks..

Re: last 256MB of the linux 32bit addresses space on p4080ds arch

yipingwang — Tue, 29 Oct 2019 09:16:12 GMT

Linux can only access 768M bytes memory which is defined by CONFIG_LOWMEM_SIZE.

CONFIG_LOWMEM_SIZE=0x30000000

CONFIG_PAGE_OFFSET=0xc0000000

CONFIG_KERNEL_START=0xc0000000

CONFIG_PHYSICAL_START=0x00000000

CONFIG_TASK_SIZE=0xc000000

You could adjust CONFIG_KERNEL_START as 0xd0000000 to assign 0XF0000000 - 0XFFFFFFFF as low memory.

Re: last 256MB of the linux 32bit addresses space on p4080ds arch

yoav_rubin2 — Tue, 29 Oct 2019 11:00:47 GMT

thanks for your reply but my situation is different:

I have 1.5GB RAM and disabled the highmem support.

In order to utilize the full RAM I increased the LOWMEM size to 0X60000000 and reduced the task size to 0x90000000.

My configuration:

CONFIG_LOWMEM_SIZE=0x60000000

CONFIG_PAGE_OFFSET=0x90000000

CONFIG_KERNEL_START=0x90000000

CONFIG_PHYSICAL_START - disabled.

CONFIG_TASK_SIZE=0x90000000

The 4GB of virtual address space is divided: 2.25 - User space (tasks) , 1.75 - Kernel space (low memory + reserved memory)

My question is how can I decrease the kernel reserved memory region (from 256MB to128MB) and increase the TASK_SIZE accordingly without changing the LOWMEM_SIZE.

If I'll move kernel_start the way you suggested , I'll face the same problem - user space 2.25 , kernel reserved region 0.25 , kernel lowmem 1.5. It will just change the locations of kernel start and reserved area...

Re: last 256MB of the linux 32bit addresses space on p4080ds arch

yipingwang — Thu, 31 Oct 2019 05:48:27 GMT

You could define CONFIG_KERNEL_START as 0x98000000.

Re: last 256MB of the linux 32bit addresses space on p4080ds arch

yoav_rubin2 — Thu, 31 Oct 2019 10:55:27 GMT

thanks for your reply!

just to completly understand:

4GB virtual space:

1. 0x0 - 0x9 : user apps

2. 0x9 - 0x98: ???

3. 0x98 - 0xE8 : kernels lowmem

4. 0xE8 - 0xF: ???

5. 0xF - 0xFFFFFFFF: ???

can you help me filling the blanks?