The uservec code in kernel/trampoline.S is used for 2 things:
usertrap().usertrap() in user/trap.c is to handle the exception based on the sscause number: syscall, device interrupt, timer interrupt, other faults. usertrap() then jumps to usertrapret() for configuring the trapframe head information, ex. kernel page table address, hartid ...
Then userret in the trampoline.S will do the reverse things in uservec and return to user mode by calling sret.
This lab requires us to jump to a user function when the tick number passes the given limit, and restore the context when the handler returns.
We can add some fields in process structure to store ticks, limit, and handler address. When the condition is triggered (ticks is big enough), we need to write the handler address to pc register, right?. No! This will certainly fails, because:
usertrap()is in kernel address space. We cannot use user address directly!
We need to ask usertrapret() for help, which is also in kernel space. It can help us jump back to the user space and restore the context saved in trapframe, including the pc register.
Therefore, we can change the epc register in trapframe to the handler address. And we call usertrapret(), kernel will do the right things for us.
You may have noticed that all registers restored by userret will be corrupted by the handler rountine, thus we can't safely jump back to the user code. So another question goes that where we can store the saved registers so that we can copy them back? If you have a look at proc.h, you will find there is space in the trapframe (4096) to store all registers (288), we can save them just beneath the original saving area.
Therefore, before usertrapret(), we should copy all registers to the new temporary area (including epc, the original one) and change the former epc to our handler address. Even if the handler uses many registers, we still have a copy for each one below.
When the handler is going to return, it is forced to add a system call sigreturn, returning to kernel space. In this system call, we restore the original context by simply copying back the memory below and calling usertrapret(). Where are we going to now? Remember, the original epc register is the user pc when the interrupt occurs.
Add a lock flag in process struct, and initialize it to “unlock” state in allocproc().