ARM Linux 进程调度（3）

switch_mm中是进行页表的切换，即将下一个的pgd的开始物理地址放入CP15中的C2寄存器。进程的pgd的虚拟地址存放在task_struct结构中的pgd指针中，通过__virt_to_phys宏可以转变成成物理地址。

static inline void

switch_mm(struct mm_struct *prev, struct mm_struct *next,

struct task_struct *tsk, unsigned int cpu)

{

if (prev != next)

cpu_switch_mm(next->pgd, tsk);

}

#define cpu_switch_mm(pgd,tsk) cpu_set_pgd(__virt_to_phys((unsigned long)(pgd)))

#define cpu_get_pgd() ({ unsigned long pg; __asm__("mrc p15, 0, %0, c2, c0, 0" : "=r" (pg)); pg &= ~0x3fff; (pgd_t *)phys_to_virt(pg); })

switch_to()完成进程上下文的切换，通过调用汇编函数__switch_to完成，其实现比较简单，也就是保存prev进程的上下文信息，该上下文信息由context_save_struct结构描述，包括主要的寄存器，然后将next的上下文信息读出，信息保存在task_struct中的thread.save中TSS_SAVE标识了thread.save在task_struct中的位置。

/*

* Register switch for ARMv3 and ARMv4 processors

* r0 = previous, r1 = next, return previous.

* previous and next are guaranteed not to be the same.

*/

ENTRY(__switch_to)

stmfd sp!, {r4 - sl, fp, lr} @ Store most regs on stack

mrs ip, cpsr

str ip, [sp, #-4]! @ Save cpsr_SVC

str sp, [r0, #TSS_SAVE] @ Save sp_SVC

ldr sp, [r1, #TSS_SAVE] @ Get saved sp_SVC

ldr r2, [r1, #TSS_DOMAIN]

ldr ip, [sp], #4

mcr p15, 0, r2, c3, c0 @ Set domain register

msr spsr, ip @ Save tasks CPSR into SPSR for this return

ldmfd sp!, {r4 - sl, fp, pc}^ @ Load all regs saved previously

struct context_save_struct {

unsigned long cpsr;

unsigned long r4;

unsigned long r5;

unsigned long r6;

unsigned long r7;

unsigned long r8;

unsigned long r9;

unsigned long sl;

unsigned long fp;

unsigned long pc;

};