Lec6- Isolation and syscall

Supervisor registers

  • stap – Store the page table’s base address
  • stvec – Store the address of trap program – trampolines, note that this address is mapped on user’s page table but without PTE_U which means the user can’t modify it.
  • sepc – Store the address in the user mode when ecall happens
  • sstrach – Store the address of trapframe which stores a frame useds to temporarily keep the user’s registers. Also used as swap “transfer station” register.
    • See the usage of csrrw a0, sstrach, a0 in the trampoline.S for some examples.

Work Flow

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						user_syscall()
						    ecall
------------------------------↓----------------------------------
						uservec-trampoline        userret()
							  ↓                     ↑
						  usertrap()           usertrapret()
						      ↓                     ↑
						   syscall()   --------------
						     ↓  ↑
					    specific SYS_call()

Some interesting features:

  • When user functions use ecall trying to enter kernel mode, what the RISCV actually does is just fetch the address at stvec and jump to that address and execute.

    • Ecall fetches the value from stvec and saves the pc in the sepc.

    • You shall see the page table haven’t been changed after ecall.

    • stvec register keep the address of trampoline

    • The last two line in the page table are exactly the kernel mode’s code, but mapped at user’s page table. Note PTE_U are not set, so user mode can’t access them.

      ![image-20211223103538097](Lec5– Trap/lec5-1.png)

  • trampoline are the two-direction bridge for both user and kernel, because its physical address for user and kernel are both map to the same virtual address.

    • Based on above handy design, we could do satp change and save 32 register into trapframe.

    • Note that sstrach initially store the address of trapframe then swap the content with a0.

    • Finally load kernel stack pointer and usertrap address then jump to kernel c code.

  • trap.c checks the reason of the trap and redirect the following trap to jump tp kernelvec instead of trampoline. Then it analysis the cause number (in lecture’s example, we use syscall, so # is 8)

    • For syscall, what usertrap() does is jump to syscall() function and syscall() will change the trapframe->a0’s value to make it received by user as the return value.