diff options
| -rw-r--r-- | kernel/trampoline.S | 31 | ||||
| -rw-r--r-- | kernel/trap.c | 6 |
2 files changed, 18 insertions, 19 deletions
diff --git a/kernel/trampoline.S b/kernel/trampoline.S index 2ce4886..0aaa413 100644 --- a/kernel/trampoline.S +++ b/kernel/trampoline.S @@ -1,18 +1,19 @@ - # - # code to switch between user and kernel space. # - # this code is mapped at the same virtual address - # (TRAMPOLINE) in user and kernel space so that - # it continues to work when it switches page tables. - # - # kernel.ld causes this to be aligned - # to a page boundary. + # low-level code to handle traps from user space into + # the kernel, and returns from kernel to user. + # + # the kernel maps the page holding this code + # at the same virtual address (TRAMPOLINE) + # in user and kernel space so that it continues + # to work when it switches page tables. + # kernel.ld causes this code to start at + # a page boundary. # #include "riscv.h" #include "memlayout.h" - .section trampsec +.section trampsec .globl trampoline trampoline: .align 4 @@ -31,7 +32,7 @@ uservec: # each process has a separate p->trapframe memory area, # but it's mapped to the same virtual address - # (TRAPFRAME) in every process. + # (TRAPFRAME) in every process's user page table. li a0, TRAPFRAME # save the user registers in TRAPFRAME @@ -70,29 +71,27 @@ uservec: csrr t0, sscratch sd t0, 112(a0) - # restore kernel stack pointer from p->trapframe->kernel_sp + # initialize kernel stack pointer, from p->trapframe->kernel_sp ld sp, 8(a0) # make tp hold the current hartid, from p->trapframe->kernel_hartid ld tp, 32(a0) - # load the address of usertrap(), p->trapframe->kernel_trap + # load the address of usertrap(), from p->trapframe->kernel_trap ld t0, 16(a0) - # restore kernel page table from p->trapframe->kernel_satp + # load the kernel page table, from p->trapframe->kernel_satp ld t1, 0(a0) csrw satp, t1 sfence.vma zero, zero - # a0 is no longer valid, since the kernel page - # table does not specially map p->tf. - # jump to usertrap(), which does not return jr t0 .globl userret userret: # userret(pagetable) + # called by usertrapret() in trap.c to # switch from kernel to user. # a0: user page table, for satp. diff --git a/kernel/trap.c b/kernel/trap.c index 75fb3ec..524da44 100644 --- a/kernel/trap.c +++ b/kernel/trap.c @@ -60,8 +60,8 @@ usertrap(void) // but we want to return to the next instruction. p->trapframe->epc += 4; - // an interrupt will change sstatus &c registers, - // so don't enable until done with those registers. + // an interrupt will change sepc, scause, and sstatus, + // so enable only now that we're done with those registers. intr_on(); syscall(); @@ -101,7 +101,7 @@ usertrapret(void) w_stvec(trampoline_uservec); // set up trapframe values that uservec will need when - // the process next re-enters the kernel. + // the process next traps into the kernel. p->trapframe->kernel_satp = r_satp(); // kernel page table p->trapframe->kernel_sp = p->kstack + PGSIZE; // process's kernel stack p->trapframe->kernel_trap = (uint64)usertrap; |