19、 Xv6 context switching (implementation of context switching; encapsulation and recovery of state machine)

One 、 Processor virtualization

Why can't an infinite loop cause the computer to jam completely ？

On the principle of ：

1、 The hardware will be interrupted （ Be similar to “ Forced insertion ” Of ecall（rics-v System call , Similar to x86 Of syscall））

2、 Switch to operating system code execution

3、 The operating system code can switch to another process for execution

Process library ：

co_yield(); // Switch coroutines

Two 、 virtualization ： State machine management

Register group （$x0...$x31,$pc）, There is only one copy of physical memory

1、 Register virtualization ： Save register state to memory

2、 Memory virtualization ：$satp Data structure of (Supervisor Address Translation and Protection Register,supervisor Address translation and protection register in mode )

Operating system code is the most important invariant( Single processor )

1、 The operating system starts processing system calls / interrupt , The state of all processes will be saved to memory

struct page{int prot, void *va, *pa;}
struct proc
{
    uint64_t x1, x2, ..., x31;
    struct page pages[MAXPAGS];
};

（1） preservation

hold x1, x2, ..., x31 Save to the current proc that will do

（2） recovery

hold pags Deliver to $stap In the corresponding data structure

3、 ... and 、 Status sealing ： Architecture related processing

x86-64

1、 interrupt / Exceptions are accompanied by stack switching

（1） adopt TSS To specify a ” Kernel stack “

The register before interrupt is saved on the stack （ Typical CISC Behavior ）

xv6（ Not limited to, RISC-V）

1、 Put the process of trap trame Assigned to a fixed address （ adopt $stap）

（1）trap frame Save to $sscratch

2、 After saving, switch to the kernel thread for execution （ Including stack switching ）