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slightly better comments

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This commit is contained in:
Robert Morris 2022-08-22 13:49:15 -04:00
parent f2ee8690ad
commit 63ef3b8c9f
2 changed files with 18 additions and 19 deletions
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29
kernel/trampoline.S
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@ -1,18 +1,19 @@
# #
# code to switch between user and kernel space. # low-level code to handle traps from user space into
# the kernel, and returns from kernel to user.
# #
# this code is mapped at the same virtual address # the kernel maps the page holding this code
# (TRAMPOLINE) in user and kernel space so that # at the same virtual address (TRAMPOLINE)
# it continues to work when it switches page tables. # in user and kernel space so that it continues
# # to work when it switches page tables.
# kernel.ld causes this to be aligned # kernel.ld causes this code to start at
# to a page boundary. # a page boundary.
# #
#include "riscv.h" #include "riscv.h"
#include "memlayout.h" #include "memlayout.h"
.section trampsec .section trampsec
.globl trampoline .globl trampoline
trampoline: trampoline:
.align 4 .align 4
@ -31,7 +32,7 @@ uservec:
# each process has a separate p->trapframe memory area, # each process has a separate p->trapframe memory area,
# but it's mapped to the same virtual address # but it's mapped to the same virtual address
# (TRAPFRAME) in every process. # (TRAPFRAME) in every process's user page table.
li a0, TRAPFRAME li a0, TRAPFRAME
# save the user registers in TRAPFRAME # save the user registers in TRAPFRAME
@ -70,29 +71,27 @@ uservec:
csrr t0, sscratch csrr t0, sscratch
sd t0, 112(a0) 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) ld sp, 8(a0)
# make tp hold the current hartid, from p->trapframe->kernel_hartid # make tp hold the current hartid, from p->trapframe->kernel_hartid
ld tp, 32(a0) 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) 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) ld t1, 0(a0)
csrw satp, t1 csrw satp, t1
sfence.vma zero, zero 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# a0 is no longer valid, since the kernel page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 # jump to usertrap(), which does not return
jr t0 jr t0
.globl userret .globl userret
userret: userret:
# userret(pagetable) # userret(pagetable)
# called by usertrapret() in trap.c to
# switch from kernel to user. # switch from kernel to user.
# a0: user page table, for satp. # a0: user page table, for satp.

6
kernel/trap.c
View file

@ -60,8 +60,8 @@ usertrap(void)
// but we want to return to the next instruction. // but we want to return to the next instruction.
p->trapframe->epc += 4; p->trapframe->epc += 4;
// an interrupt will change sstatus &c registers, // an interrupt will change sepc, scause, and sstatus,
// so don't enable until done with those registers. // so enable only now that we're done with those registers.
intr_on(); intr_on();
syscall(); syscall();
@ -101,7 +101,7 @@ usertrapret(void)
w_stvec(trampoline_uservec); w_stvec(trampoline_uservec);
// set up trapframe values that uservec will need when // 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_satp = r_satp(); // kernel page table
p->trapframe->kernel_sp = p->kstack + PGSIZE; // process's kernel stack p->trapframe->kernel_sp = p->kstack + PGSIZE; // process's kernel stack
p->trapframe->kernel_trap = (uint64)usertrap; p->trapframe->kernel_trap = (uint64)usertrap;