ARM V7 切换上下文的规范

在ARM Linux中，在切换用户进程的时候，会切换页目录表。对于V7的Processor，切换的具体的代码是：

41 ENTRY(cpu_v7_switch_mm)

 42 #ifdef CONFIG_MMU

 43         mov     r2, #0

 44         ldr     r1, [r1, #MM_CONTEXT_ID]        @ get mm->context.id

 45         ALT_SMP(orr     r0, r0, #TTB_FLAGS_SMP)

 46         ALT_UP(orr      r0, r0, #TTB_FLAGS_UP)

 47 #ifdef CONFIG_ARM_ERRATA_430973

 48         mcr     p15, 0, r2, c7, c5, 6           @ flush BTAC/BTB

 49 #endif

 50 #ifdef CONFIG_ARM_ERRATA_754322

 51         dsb

 52 #endif

 53         mcr     p15, 0, r2, c13, c0, 1          @ set reserved context ID

 54         isb @ flush the pipeline

 55 1:      mcr     p15, 0, r0, c2, c0, 0           @ set TTBR 0

 56         isb

 57 #ifdef CONFIG_ARM_ERRATA_754322

 58         dsb

 59 #endif

 60         mcr     p15, 0, r1, c13, c0, 1          @ set context ID

 61         isb

 62 #endif

 63         mov     pc, lr

 64 ENDPROC(cpu_v7_switch_mm)

44行，从task_struct中获取要调度进程的contextID

45，46，准备PGD的属性

53～56，写入TTBR0

60行，写入新的contextID.

63行，返回

在这个函数中，有疑问的地方在于53行，为什么要在写入新的TTBR0之前，要先把contextID的register写入0。在ARM reference中，是这样写的：

Synchronization is necessary to avoid either:

• the old ASID being associated with translation table walks from the new translation tables

• the new ASID being associated with translation table walks from the old translation tables.

在ARM的reference中，举了3个可以避免冲突的例子。这三个例子如下：

Change ASID to 0

ISB

Change Translation Table Base Register

ISB

Change ASID to new value

Change Translation Table Base Register to the global-only mappings

ISB

Change ASID to new value

ISB

Change Translation Table Base Register to new value

Set TTBCR.PD0 = 1

ISB

Change ASID to new value

Change Translation Table Base Register to new value

ISB

Set TTBCR.PD0 = 0