Memory Model ############### `AArch64 memory model `_ 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_GRE - Device_nGRE - Device_nGnRE - Device_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