Arm: Memory Model
- doc
- memory
- memory access ordering
- memory types
- permissions attributes
- Access Flag
- memory aliasing and mismatched memory types
doc
Memory address: peripherals, kernel data, kernel code, application data, application code
memory
每个memory translation table (page table) entry，称为block, or page descriptor.
block descriptor：
- Uppper Attributes: Reserverd for software use, UXN (执行权限), PXN (执行权限), Contig
- Output Block Address
- Lower Attributes: nG, AF, SH(share), AP(access permission), NS, Indx
hierarchical attributes
由于虚拟地址转换，memory attributes是follow更底层（或者说更高权限的）的设定
Peripherals: Non-cached Ordered, R/W Not executable, Privileged access only
Kernel Data: Cacheable, R/W Not executable, Privileged access only
Kernel Code: Cacheable, Read-only executable, Privileged access only
Application Data: Cacheable, R/W Not executable, Unprivileged
Application code: Cacheable, Read-only executable, Unprivileged
memory access ordering
SSE: Simple Sequential Execution，指令按顺序执行
注意：memory access ordering 与 instructions ordering 不同
memory types
包含memory type，以及cacheability information
normal memory, device memory
normal memory
现代处理器可能merge accesses（提升性能）、predicting behavior（预取，提升速度）
device memory
device memory for peripherals: Memory Mapped I/O (MMIO)
Device_GREDevice_nGREDevice_nGnREDevice_nGnRnE
Gathering(G, nG): 是否允许merging multiple access
Re-ordering(R, nR)：是否允许re-order到某个peripheral的accesses
Early Write Acknowledgement(E, nE): 什么时候认为一个write是完成的，是到cache认为完成，还是到目标地址认为完成
permissions attributes
PAN (Privileged Access Never): APP可以指定某些内容禁止OS特权LDR访问。注意，挡不住OS的LDTR/STTR特权访问。
UXN (User Execute Never): EL0
PXN (Privileged Execute Never): EL3/EL2
- Peripherals: Not executable, PXN=1, UXN=1
- Kernel Data: Not executable, PXN=1, UXN=1
- Kernel Code: Privileged executable, PXN=0, UXN=1
- Application Data: Not executable, PXN=1, UXN=1
- Application Code: Unprivileged executable, PXN=1, UXN=0
A location with EL0 write permissions is never executable at EL1 => 关键
Device regions: Execute Never (XN)
Access Flag
AF = 0: Region not accessed
AF = 1: Region accessed
memory aliasing and mismatched memory types
multiple virtual addresses => aliasing
如果virtual address space在stage1 tables标记为device，在stage2 tables标记为normal，那么从严不从宽，视为Device。
如果HCR_EL2.FWB置位，则stage2 tables的标记可以覆盖任何stage1 tables的标记
