[mit 6.s081] LEC 6: isolation & system call entry/exit notes

Lec 6: Isolation & system call entry/exit

• Ref: https://github.com/huihongxiao/MIT6.S081/tree/master/lec06-isolation-and-system-call-entry-exit-robert
• Preparation: xv6 book Chapter 4 except 4.6

Trap

• trap: Switching between user space and kernel space
• Switch timing :
  • system call
  • abnormal Exception, In case of page missing error (page fault), Divide by zero error
  • Device triggered interrupt
• Important registers :
  • STVEC(Supervisor Trap Vector Base Address Register): Point to processing in the kernel trap The first address of the instruction ( According to the user or kernel , The code will be different )
  • SEPC(Supervisor Exception Program Counter): trap Save the value of the program counter in the process
  • SSRATCH(Supervisor Scratch Register): To hold trapframe Page virtual address
• Operations to be completed during switching :
  • preservation 32 User registers
  • Save program counter PC
  • Transfer mode MODE Switch from user state to kernel state
  • take SATP The register points from the user page table to the kernel page table
  • You need a stack to call the kernel C function , And make SP Point to
  • Jump into kernel C Code
• Kernel mode (supervisor mode) The privilege of :
  • Reading and writing SATP register , Yes STVEC, SEPC, SSCRATCH Wait for the register to operate
  • have access to PTE_U Sign bit is 0 Of PTE

Trap Code execution process

1. System call through ECALL Instructions switch to the kernel ( This instruction does not switch page tables , There is no user page table at this time )
2. The kernel executes assembly functions uservec, yes trampoline.S in trampoline Part of . This function switches SSCRATCH and a0 Register value , after a0 The value is trapframe The address of , And then 32 The values of user registers and other registers are stored in p->trapframe in ; Then the kernel stack frame pointer will be loaded (kernel_sp), And load the address of the kernel page table to SATP register Complete the switching of the kernel page table . Finally jump to C Code trap.c Of usertrap()
3. usertrap() First of all, will STVEC The register is made up of usertrap Change to kerneltrap( Because sending interrupts again at this time belongs to having interrupts from the kernel trap). Then, if it is a system call, call syscall() Function to find the function of the corresponding system function from the system call table sys_xxx() To perform , Go back to syscall(); If it is an interrupt from the device, execute devintr. In the end, it will call usertrapret().
4. perform trap.c Medium usertrapret(). First of all, I will p->trapframe Set the values of the fields related to the kernel , For the next time trap. Set up SSTATUS User mode flag bit . Finally, call the assembly function userret.
5. perform trampoline.S Medium userret function . First switch back to the user page table , And then from trapframe Load the value into the register , The final will be trapframe The address is stored in the register SSCRATCH in .
6. Back to user space .
   

• ECALL effect :
  • Change the code from user mode to kernel mode
  • take PC To preserve with SEPC
  • Jump to STVEC Register points to ( Handle trap Of ) Instructions
• How to save user registers :
  • The kernel will trapframe Mapped to each user page table
  • Before entering user space ( The operating system starts in kernel mode ), The kernel will trapframe The address is saved at SSCRATCH register , coordination csrrw Instruction register a0 and sscratch Content exchange .