On Tue, Nov 25, 2014 at 02:16:07PM +0000, zhichang.yuan wrote:
On 2014年11月25日 01:17, Catalin Marinas wrote:
I'm trying to make some sense of this patch, so questions below:
On Wed, Nov 19, 2014 at 02:21:55PM +0000, zhichang.yuan@linaro.org wrote:
From: "zhichang.yuan" zhichang.yuan@linaro.org
This patch make the processing of map_mem more common and support more discrete memory layout cases.
In current map_mem, the processing is based on two hypotheses:
- no any early page allocations occur before the first PMD or PUD regime
where the kernel image locate is successfully mapped;
No because we use the kernel load offset to infer the start of RAM (PHYS_OFFSET). This would define which memory you can allocate.
I note that the current PHYS_OFFSET is 0x8000,0000 in JUNO, 0x4000,0000 in QEMU. I think the current processing like that: the booloader load the kernel image at (PHYS_OFFSET + TEXT_OFFSET), and vmlinux.lds.S define the VMA of image as (. = PAGE_OFFSET + TEXT_OFFSET). So, the starting RAM physical address, PHYS_OFFSET, correspond to PAGE_OFFSET now.( this is my inference, have not investigate the UEFI)
Yes, but the PAGE_OFFSET / PHYS_OFFSET relation is true on any architecture. The linear kernel mapping translates virtual address at PAGE_OFFSET to physical address PHYS_OFFSET.
Also note that PHYS_OFFSET is not known at build time. The kernel entry code calculates it by subtracting TEXT_OFFSET from its load address. TEXT_OFFSET is known at build time.
But is it possible in the future the kernel image is loaded to a memory range that is not the first memblock, such as :
block 0: 0x100000, 0x20100000 block 1: 0x40000000, 0x40000000
Supposed the block 1 is where the kernel image locate.
Actually, if bootloader put the kernel image at a configurable physical address named as PA, and VMA of text section is defined as PAGE_OFFSET + 0x100000 + PA, then PAGE_OFFSET will correspond to 0x100000.
Basically what you want is configurable TEXT_OFFSET based on your configurable physical address PA. For single kernel Image running on multiple platforms, we don't want this. PA in this case would be platform specific.
In x86, the VMA of text section is as below:
#ifdef CONFIG_X86_32 . = LOAD_OFFSET + LOAD_PHYSICAL_ADDR; #else . = __START_KERNEL; #endif
LOAD_PHYSICAL_ADD is configurable. I think it can support different hardware design.
One x86, this LOAD_PHYSICAL_ADDR is defined to CONFIG_PHYSICAL_START which is only enabled if EXPERT || CRASH_DUMP. It's not a general user config option for exactly the same reasons as stated above - single kernel Image.
- there are sufficient available pages in the PMD or PUD regime to satisfy
the need of page tables from other memory ranges mapping.
I don't fully understand this. Can you be more specific?
Supposed this memory layout:
block 0: 0x40000000, 0xc00000 block 1: 0x60000000, 0x1f000000 block 2: 0x80000000, 0x40000000
if the end of kernel image is near to 0xc00000, it is possible no available mapped pages for other blocks mapping.
Of-course, this is a very special case, not practical, since the memblock where the kernel image locate should be big enough.
So is this a real use-case?
In addition, for the 4K page system, to comply with the constraint No.1, the start address of some memory ranges is forced to align at PMD boundary, it will make some marginal pages of that ranges are skipped to build the PTE. It is not reasonable.
It is reasonable to ask for the start of RAM to be on a PMD (2MB) boundary.
I think the physical address where the kernel image locate can be limited on PMD boundary. But the start of RAM is decided by Soc or hardware platform. For example, the start of RAM only align to MB boundary.
But, again, do you have a real use-case in mind or just theoretical? For arm64, we expect at least a bit of alignment with the SBSA where the memory starts on a GB boundary.
This patch will relieve the system from those constraints. You can load the kernel image in any memory range, the memory range can be small, can start at non-alignment boundary, and so on.
I guess you still depend on the PAGE_OFFSET, TEXT_OFFSET, so it's not random.
I'm not sure what the end goal is with this patch but my plan is to entirely decouple TEXT_OFFSET from PAGE_OFFSET (with a duplicate mapping for the memory covering the kernel text). This would allow us to load the kernel anywhere in RAM (well, with some sane alignment to benefit from section mapping) and the PHYS_OFFSET detected from DT at run-time. Once that's done, I don't think your patch is necessary.
I am not so clear what is the coupling between TEXT_OFFSET and PAGE_OFFSET. It seems the VMA and LMA have some coupling.
(I don't entirely follow the VMA and LMA acronyms, something to do with virtual address and load address?)
PHYS_OFFSET + TEXT_OFFSET <------------> PAGE_OFFSET + TEXT_OFFSET.
What I meant is that we should no longer mandate that the kernel Image is loaded at PHYS_OFFSET + TEXT_OFFSET.
With additional kernel changes it could be loaded at (TEXT_OFFSET + random-2MB-aligned-address) which gets mapped during boot to a KERNEL_PAGE_OFFSET, different from PAGE_OFFSET. We still have the PAGE_OFFSET -> PHYS_OFFSET correspondence but not with KERNEL_PAGE_OFFSET. TEXT_OFFSET, PAGE_OFFSET and KERNEL_PAGE_OFFSET would be build-time configurations while PHYS_OFFSET would be computed at run-time based on the memory blocks described in DT (rather than kernel-load-addr - TEXT_OFFSET).