With x86 32-bit virtual address space and lower physical memory mapped continuousely after kernel at 0xc0000000 the upper physical memory part needed to be mapped into the virtual address space dynamically.
Has this changed in the x86_64 kernel?
Is there still HIGHMEM allocation or is all phyical memory in x86_64 accessible with simple physical to virtual address translation macro?
No. The highmem comes from ZONE_DMA、ZONE_NORMAL and ZONE_HIGHMEM. But in 64, cause it's really huge, we split the kernel spaces into several part with large holes between them for safe, and there are nothing called high memory there. You can read this for more detail about the structure of x64 kernel address.
I found this one: https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt
ff11000000000000 | -59.75 PB | ff90ffffffffffff | 32 PB | direct mapping of all physical memory (page_offset_base)
