6.S081 Xv6 Lab 4 traps
2024-06-03 12:20:18
Lab: traps
6.S081 的 Xv6 RISC-V Lab traps，实验内容：
- https://pdos.csail.mit.edu/6.S081/2020/labs/traps.html
$ git fetch
$ git checkout traps
$ make clean
RISC-V assembly
这题没什么具体要做的，就看一看，跟着题目学一下 RISC- V 汇编。
Backtrace
这题主要是实现打印函数栈。就是 GDB 里面 bt 的这种效果：
(gdb) bt
#0 fork () at kernel/proc.c:260
#1 0x0000000080002c3c in sys_fork () at kernel/sysproc.c:29
#2 0x0000000080002bb0 in syscall () at kernel/syscall.c:140
#3 0x000000008000289a in usertrap () at kernel/trap.c:67
#4 0x0000000000000050 in ?? ()
关键代码
kernel/riscv.h: 实现一个获取当前 frame pointer 的方法：
static inline uint64
r_fp()
{uint64 x;asm volatile("mv %0, s0" : "=r" (x) );return x;
}
kernel/printf.c: 打印 backtrace：
void
backtrace(void)
{printf("backtrace:\n");uint64 fp = r_fp();while (PGROUNDDOWN(fp) < PGROUNDUP(fp)) {printf("%p\n", *(uint64*)(fp-8));fp = *(uint64*)(fp-16);}
}
Diff
diff --git a/kernel/defs.h b/kernel/defs.h
index 4b9bbc0..137c786 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -80,6 +80,7 @@ int pipewrite(struct pipe*, uint64, int);void printf(char*, ...);void panic(char*) __attribute__((noreturn));void printfinit(void);
+void backtrace(void);// proc.cint cpuid(void);
diff --git a/kernel/printf.c b/kernel/printf.c
index e1347de..fbdeb68 100644
--- a/kernel/printf.c
+++ b/kernel/printf.c
@@ -121,6 +121,9 @@ panic(char *s)printf("panic: ");printf(s);printf("\n");
+
+ backtrace();
+panicked = 1; // freeze uart output from other CPUsfor(;;);
@@ -132,3 +135,18 @@ printfinit(void)initlock(&pr.lock, "pr");pr.locking = 1;}
+
+// print a backtrace:
+// a list of the function calls on the stack above
+// the point at which the error occurred.
+void
+backtrace(void)
+{
+ printf("backtrace:\n");
+ uint64 fp = r_fp();
+ while (PGROUNDDOWN(fp) < PGROUNDUP(fp)) {
+ printf("%p\n", *(uint64*)(fp-8));
+ fp = *(uint64*)(fp-16);
+ }
+}
+
diff --git a/kernel/riscv.h b/kernel/riscv.h
index 0aec003..c95316e 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -319,6 +319,16 @@ sfence_vma()asm volatile("sfence.vma zero, zero");}+// compiler stores the frame pointer of the currently
+// executing function in s0.
+// this function reads current frame pointer, namely s0
+static inline uint64
+r_fp()
+{
+ uint64 x;
+ asm volatile("mv %0, s0" : "=r" (x) );
+ return x;
+}#define PGSIZE 4096 // bytes per page#define PGSHIFT 12 // bits of offset within a page
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index e8bcda9..a520959 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -58,6 +58,8 @@ sys_sleep(void)int n;uint ticks0;+ backtrace();
+if(argint(0, &n) < 0)return -1;acquire(&tickslock);
Alarm
做一套系统调用，在用户进程每运行指定次数时钟后，调用该进程提供的一个 handler。
其实也不难，（我觉得比页表简单），跟着 hint 一步步做就行了。
关键实现
kernel/proc.h: 在 proc 结构体里添加需要的字段（间隔时长、已运行时长计数、处理函数地址(vm)、保存 alarm 前的寄存器）：
struct proc {...int alarm_interval; // the alarm interval (ticks)int alarm_passed; // how many ticks have passed since the last calluint64 alarm_handler; // pointer to the alarm handler functionstruct trapframe etpfm; // trapframe to resume
}
- 添加
sys_sigalarm系统调用：接收参数，为进程设置 alarm 的间隔时长和处理函数。最终的实现在kernel/sysproc.c（添加新系统调用的完整步骤见：6.S081 Xv6 Lab 2: system calls）:
uint64
sys_sigalarm(void)
{int interval;uint64 handler;if(argint(0, &interval) < 0)return -1;if(argaddr(1, &handler) < 0)return -1;struct proc *p = myproc();p->alarm_interval = interval;p->alarm_handler = handler;return 0;
}
- 在
usertrap（kernel/trap.c）中处理时钟中断时，如果进程需要 alarm 就保存当前 trapframe 到 etpfm，调用 handler （把 handler 地址放到 trapframe->epc，回到用户空间之后就会运行该函数）：
void
usertrap(void)
{...if(which_dev == 2) {// alarmif (p->alarm_interval) {if (++p->alarm_passed == p->alarm_interval) {memmove(&(p->etpfm), p->trapframe, sizeof(struct trapframe));// return to alarm handler: call p->alarm_handler();p->trapframe->epc = p->alarm_handler;}}yield();}...
}
- 实现
sys_sigalarm系统调用，恢复 alarm 前的 trapframe（回到用户空间就会接着 alarm 之前的 PC 开始运行），把 alarm_passed 计数器置为零（允许下一次 alarm）：
uint64
sys_sigreturn(void)
{struct proc *p = myproc();memmove(p->trapframe, &(p->etpfm), sizeof(struct trapframe));p->alarm_passed = 0;return 0;
}
某bug的sizeof
我做的时候不小心写了个 sizeof 的语法错误，找了半个小时呢。。。（这个问题超初学者的，，我对8起 K&R）
我一开始是这么写的：
struct trapframe {...}struct proc {...struct trapframe *trapframe;struct trapframe etpfm;
}void
usertrap(void)
{...memmove(&(p->etpfm), p->trapframe, sizeof(p->trapframe));...
}uint64
sys_sigreturn(void)
{...memmove(p->trapframe, &(p->etpfm), sizeof(p->trapframe));...
}
解决：把 sizeof(p->trapframe) 改成 sizeof(struct trapframe)。
Diff
emmm，上一题做完忘记 commit 了，所以下面这个 diff 也包含了上一题（backtrace）的：
diff --git a/Makefile b/Makefile
index 1fa367e..a74296b 100644
--- a/Makefile
+++ b/Makefile
@@ -175,6 +175,7 @@ UPROGS=\$U/_grind\$U/_wc\$U/_zombie\
+ $U/_alarmtest\diff --git a/grade-lab-traps b/grade-lab-traps
index 058e77b..8619bbe 100755
--- a/grade-lab-traps
+++ b/grade-lab-traps
@@ -60,7 +60,7 @@ def test_alarmtest_test2():def test_usertests():r.run_qemu(shell_script(['usertests'
- ]), timeout=300)
+ ]), timeout=500)r.match('^ALL TESTS PASSED$')@test(1, "time")
diff --git a/kernel/defs.h b/kernel/defs.h
index 4b9bbc0..137c786 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -80,6 +80,7 @@ int pipewrite(struct pipe*, uint64, int);void printf(char*, ...);void panic(char*) __attribute__((noreturn));void printfinit(void);
+void backtrace(void);// proc.cint cpuid(void);
diff --git a/kernel/printf.c b/kernel/printf.c
index e1347de..fbdeb68 100644
--- a/kernel/printf.c
+++ b/kernel/printf.c
@@ -121,6 +121,9 @@ panic(char *s)printf("panic: ");printf(s);printf("\n");
+
+ backtrace();
+panicked = 1; // freeze uart output from other CPUsfor(;;);
@@ -132,3 +135,18 @@ printfinit(void)initlock(&pr.lock, "pr");pr.locking = 1;}
+
+// print a backtrace:
+// a list of the function calls on the stack above
+// the point at which the error occurred.
+void
+backtrace(void)
+{
+ printf("backtrace:\n");
+ uint64 fp = r_fp();
+ while (PGROUNDDOWN(fp) < PGROUNDUP(fp)) {
+ printf("%p\n", *(uint64*)(fp-8));
+ fp = *(uint64*)(fp-16);
+ }
+}
+
diff --git a/kernel/proc.c b/kernel/proc.c
index dab1e1d..237fecb 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -127,6 +127,11 @@ found:p->context.ra = (uint64)forkret;p->context.sp = p->kstack + PGSIZE;+ // Initialize alarm
+ p->alarm_interval = 0;
+ p->alarm_passed = 0;
+ p->alarm_handler = 0;
+return p;}diff --git a/kernel/proc.h b/kernel/proc.h
index 9c16ea7..2dac44f 100644
--- a/kernel/proc.h
+++ b/kernel/proc.h
@@ -103,4 +103,9 @@ struct proc {struct file *ofile[NOFILE]; // Open filesstruct inode *cwd; // Current directorychar name[16]; // Process name (debugging)
+
+ int alarm_interval; // the alarm interval (ticks)
+ int alarm_passed; // how many ticks have passed since the last call
+ uint64 alarm_handler; // pointer to the alarm handler function
+ struct trapframe etpfm; // trapframe to resume, 啊, 不会xv6的动态内存分配，所以这里直接 hardcode 一个对象了。};
diff --git a/kernel/riscv.h b/kernel/riscv.h
index 0aec003..c95316e 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -319,6 +319,16 @@ sfence_vma()asm volatile("sfence.vma zero, zero");}+// compiler stores the frame pointer of the currently
+// executing function in s0.
+// this function reads current frame pointer, namely s0
+static inline uint64
+r_fp()
+{
+ uint64 x;
+ asm volatile("mv %0, s0" : "=r" (x) );
+ return x;
+}#define PGSIZE 4096 // bytes per page#define PGSHIFT 12 // bits of offset within a page
diff --git a/kernel/syscall.c b/kernel/syscall.c
index c1b3670..eb079af 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -104,6 +104,8 @@ extern uint64 sys_unlink(void);extern uint64 sys_wait(void);extern uint64 sys_write(void);extern uint64 sys_uptime(void);
+extern uint64 sys_sigalarm(void);
+extern uint64 sys_sigreturn(void);static uint64 (*syscalls[])(void) = {[SYS_fork] sys_fork,
@@ -127,6 +129,8 @@ static uint64 (*syscalls[])(void) = {[SYS_link] sys_link,[SYS_mkdir] sys_mkdir,[SYS_close] sys_close,
+[SYS_sigalarm] sys_sigalarm,
+[SYS_sigreturn] sys_sigreturn,};void
diff --git a/kernel/syscall.h b/kernel/syscall.h
index bc5f356..382d781 100644
--- a/kernel/syscall.h
+++ b/kernel/syscall.h
@@ -20,3 +20,5 @@#define SYS_link 19#define SYS_mkdir 20#define SYS_close 21
+#define SYS_sigalarm 22
+#define SYS_sigreturn 23
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index e8bcda9..5b64016 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -58,6 +58,8 @@ sys_sleep(void)int n;uint ticks0;+ backtrace();
+if(argint(0, &n) < 0)return -1;acquire(&tickslock);
@@ -95,3 +97,32 @@ sys_uptime(void)release(&tickslock);return xticks;}
+
+uint64
+sys_sigalarm(void)
+{
+ int interval;
+ uint64 handler;
+
+ if(argint(0, &interval) < 0)
+ return -1;
+
+ if(argaddr(1, &handler) < 0)
+ return -1;
+
+ struct proc *p = myproc();
+
+ p->alarm_interval = interval;
+ p->alarm_handler = handler;
+
+ return 0;
+}
+
+uint64
+sys_sigreturn(void)
+{
+ struct proc *p = myproc();
+ memmove(p->trapframe, &(p->etpfm), sizeof(struct trapframe));
+ p->alarm_passed = 0;
+ return 0;
+}
diff --git a/kernel/trap.c b/kernel/trap.c
index a63249e..9f2a64b 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -77,8 +77,20 @@ usertrap(void)exit(-1);// give up the CPU if this is a timer interrupt.
- if(which_dev == 2)
+ if(which_dev == 2) {
+ // alarm
+ if (p->alarm_interval /*&& p->alarm_handler*/) { // handler addr might be 0
+ if (++p->alarm_passed == p->alarm_interval) {
+ memmove(&(p->etpfm), p->trapframe, sizeof(struct trapframe));
+ // return to alarm handler: call p->alarm_handler();
+ p->trapframe->epc = p->alarm_handler;
+ // printf("[DEBUG] alarm: %s(%d), handler=%x\n",
+ // p->name, p->pid, p->alarm_handler);
+ // p->alarm_passed = 0; // sigreturn 时再恢复: prevent re-entrant calls to the handler
+ }
+ }yield();
+ }usertrapret();}
diff --git a/user/alarmtest.c b/user/alarmtest.c
index 38f09ff..427c460 100644
--- a/user/alarmtest.c
+++ b/user/alarmtest.c
@@ -101,6 +101,7 @@ test1()// restored correctly, causing i or j or the address ofj// to get an incorrect value.printf("\ntest1 failed: foo() executed fewer times than it was called\n");
+ printf("\ti=%d, j=%d\n", i, j);} else {printf("test1 passed\n");}
diff --git a/user/user.h b/user/user.h
index b71ecda..57404e0 100644
--- a/user/user.h
+++ b/user/user.h
@@ -23,6 +23,8 @@ int getpid(void);char* sbrk(int);int sleep(int);int uptime(void);
+int sigalarm(int ticks, void (*handler)());
+int sigreturn(void);// ulib.cint stat(const char*, struct stat*);
diff --git a/user/usys.pl b/user/usys.pl
index 01e426e..fa548b0 100755
--- a/user/usys.pl
+++ b/user/usys.pl
@@ -36,3 +36,5 @@ entry("getpid");entry("sbrk");entry("sleep");entry("uptime");
+entry("sigalarm");
+entry("sigreturn");
实验结果
最后，make grade (我的机器上跑 usertests 巨慢，要改一下 timeout 才能通过。。):
== Test answers-traps.txt == answers-traps.txt: OK
== Test backtrace test ==
$ make qemu-gdb
backtrace test: OK (4.7s)
== Test running alarmtest ==
$ make qemu-gdb
(3.9s)
== Test alarmtest: test0 == alarmtest: test0: OK
== Test alarmtest: test1 == alarmtest: test1: OK
== Test alarmtest: test2 == alarmtest: test2: OK
== Test usertests ==
$ make qemu-gdb
usertests: OK (448.3s) (Old xv6.out.usertests failure log removed)
== Test time ==
time: OK
Score: 85/85
EOF
By CDFMLR 2020-03-28
顶部图片来自于小歪API，系随机选取的图片，仅用于检测屏幕显示的机械、光电性能，与文章的任何内容及观点无关，也并不代表本人局部或全部同意、支持或者反对其中的任何内容及观点。如有侵权，联系删除。
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