A virtual memory is allocated via k_mem_map() cannot be used for a partition in arm64 MMU-based?

[Dat-NguyenDuy]

Hello, I'm using newlib for arm cortex-a53 with MMU is enabled, Zephyr image is loaded and ran from SRAM only.

I want to use printf in user thread, and i noticed that it will use newlib heap: https://github.com/zephyrproject-rtos/zephyr/blob/main/lib/libc/newlib/libc-hooks.c#L290 via _sbrk.

The heap is currently alocated via k_mem_map(): https://github.com/zephyrproject-rtos/zephyr/blob/main/lib/libc/newlib/libc-hooks.c#L111 and is added to z_malloc_partition.

So to use printf in user thread, i added z_malloc_partition into a domain by using k_mem_domain_add_partition()

But i see that currently, the private map always treats the virtual memory and physical memory are the same: https://github.com/zephyrproject-rtos/zephyr/blob/main/arch/arm64/core/mmu.c#L1057-L1058

So i'm not clear that the virtual address and physical address for the partition should be the same before adding it into a domain.

With these Kconfigs from my side:

- CONFIG_SRAM_BASE_ADDRESS = CONFIG_KERNEL_VM_BASE = 0x33C80000
- CONFIG_SRAM_SIZE = 3584 (3.5 Mb)
- CONFIG_KERNEL_VM_OFFSET = 0
- CONFIG_KERNEL_VM_SIZE = 0x800000

And here is the chart how the heap is allocated, there is a risk that the heap will be mapped in to invalid SRAM after k_mem_domain_add_partition():

Based on that, I have few points want to clarify:

1. The malloc_prepare implementation for heap is wrong for arm64 MMU? because we cannot use k_mem_map() to map the heap (heap virtual memory is may difference than physical memory) and then add it to a domain (expect virtual and physical are the same).

2. Allocate a virtual region via k_mem_map() and add it in to a domain is not intended behavior, but how user space can use newlib heap with current implementation for heap?

3. CONFIG_KERNEL_VM_SIZE must be set equal or less than CONFIG_SRAM_SIZE to ensure that the virtual memory always fit in physical SRAM, in my side:

   • CONFIG_KERNEL_VM_SIZE = 0x380000 (=3584*1024)

   

Thank you!!
Dat Nguyen