Memory Management
#####################

`Memory management <https://developer.arm.com/architectures/learn-the-architecture/memory-management/single-page>`_

virtual address -> physical address 映射关系放在Translation table中

software manages the TLBs(Translation Lookaside Buffers)

memory management
====================

软件访问虚拟地址(virtual address space)，处理器转换为物理地址

通过虚拟地址机制，OS可以控制软件访问的权限，天然适合sandbox场景

virtual and physical address
================================

virtual address space: peripherals, kernel data, kernel code, application data, application code

-> Translation Tables

physical address space: ddr, peripherals, flash, sram, rom

注意，Translation Tables一般由OS/hypervisor运行软件维护，在memory中载入


Memory Management Unit (MMU)
===============================

MMU负责执行virtual address与physical address的translation：

- table walk unit: 负责执行管理逻辑，并且将Translation Tables读入memory

- Translation Lookaside Buffers (TLBs): 缓存最近的translation

即，先找TLBs，再Table Walk Unit去Memory找

Translation Tables里包含PA base, Attributes


Address spaces
==================

    NS.EL1 / NS.EL0 
    -> Guest OS Tables (stage 1 Translation) 
    -> physical memory map seen by guest os (peripherals, ram, flash) 
    -> Virtualization Tables (stage 2 Translation)
    -> (peripherals, ram, flash)

    NS.EL2 
    -> Hypervisor Virtual memory map
    -> Hypervisor Tables
    -> (peripherals, ram, flash)

    EL3
    -> Secure Monitor Virtual memory map
    -> Secure Monitor Tables
    -> (peripherals, ram, flash)

IPAs: intermediate physical addresses

virtual address space
========================

虚拟地址空间大小可以动态设定，以TCR_ELx.TnSZ动态控制：2^(64 - TCR_ELx.TnSZ)

不能设定比物理地址大的虚拟地址空间。

Address Space Identifiers（ASIDs）：用于标识不同app的虚拟地址空间。

TLB：VA Tag（标识虚拟地址空间前缀——区分Kernel Space, User Space, ...）、ASID、Descriptor

Virtual Machine Identifiers: VMID => 区分不同的VM