.. SPDX-License-Identifier: GPL-2.0

================= Memory Management

Virtual memory layout

.. note::

• Some aspects of the virtual memory layout setup are not clarified (number of page levels, alignment, DMA memory).

• Unused gaps in the virtual memory layout could be present or not - depending on how partucular system is configured. No page tables are created for the unused gaps.

• The virtual memory regions are tracked or untracked by KASAN instrumentation, as well as the KASAN shadow memory itself is created only when CONFIG_KASAN configuration option is enabled.

::

============================================================================= | Physical | Virtual | VM area description

+- 0 --------------+- 0 --------------+ | | S390_lowcore | Low-address memory | +- 8 KB -----------+ | | | | | | | | ... unused gap | KASAN untracked | | | +- AMODE31_START --+- AMODE31_START --+ .amode31 rand. phys/virt start |.amode31 text/data|.amode31 text/data| KASAN untracked +- AMODE31_END ----+- AMODE31_END ----+ .amode31 rand. phys/virt end (<2GB) | | | | | | +- __kaslr_offset_phys | kernel rand. phys start | | | | kernel text/data | | | | | +------------------+ | kernel phys end | | | | | | | | | | | | +- ident_map_size -+ | | | | ... unused gap | KASAN untracked | | +- __identity_base + identity mapping start (>= 2GB) | | | identity | phys == virt - __identity_base | mapping | virt == phys + __identity_base | | | | KASAN tracked | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | +---- vmemmap -----+ 'struct page' array start | | | virtually mapped | | memory map | KASAN untracked | | +- __abs_lowcore --+ | | | Absolute Lowcore | KASAN untracked | | +- __memcpy_real_area | | | Real Memory Copy| KASAN untracked | | +- VMALLOC_START --+ vmalloc area start | | KASAN untracked or | vmalloc area | KASAN shallowly populated in case | | CONFIG_KASAN_VMALLOC=y +- MODULES_VADDR --+ modules area start | | KASAN allocated per module or | modules area | KASAN shallowly populated in case | | CONFIG_KASAN_VMALLOC=y +- __kaslr_offset -+ kernel rand. virt start | | KASAN tracked | kernel text/data | phys == (kvirt - __kaslr_offset) + | | __kaslr_offset_phys +- kernel .bss end + kernel rand. virt end | | | ... unused gap | KASAN untracked | | +------------------+ UltraVisor Secure Storage limit | | | ... unused gap | KASAN untracked | | +KASAN_SHADOW_START+ KASAN shadow memory start | | | KASAN shadow | KASAN untracked | | +------------------+ ASCE limit | | | CONFIG_ILLEGAL_POINTER_VALUE causes memory access fault | | +------------------+