From 63ef3b8c9fd15d5ea5775813cda94a3c64cff0d3 Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Mon, 22 Aug 2022 13:49:15 -0400
Subject: slightly better comments
---
kernel/trampoline.S | 31 +++++++++++++++----------------
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- # a0 is no longer valid, since the kernel page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;
--
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