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Print a page table ok

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catfood 2023-01-22 15:46:30 +00:00
parent 74c8ae89b6
commit 1107e7bcab
5 changed files with 35 additions and 3 deletions
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3
answers-page-tables.txt
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@ -1,3 +0,0 @@
Q: Which other xv6 system call(s) could be made faster using this shared page? Explain how.
A: sys_uptime. Make the global tick in a user access page, and map the address of tick into usyscall.

8
answers-pgtbl.txt Normal file
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@ -0,0 +1,8 @@
Q: Which other xv6 system call(s) could be made faster using this shared page? Explain how.
A: sys_uptime. Make the global tick in a user access page, and map the address of tick into usyscall.
Q: Explain the output of vmprint in terms of Fig 3-4 from the text. What does page 0 contain? What is in page 2? When running in user mode, could the process read/write the memory mapped by page 1? What does the third to last page contain?
A: Page 0 contains process's text (code).
Page 2 is the guard page.
Page 1 could be read and write in user mode, due to its PTE_U(1 << 4), PTE_R(1 << 2) and PTE_W(1 << 3) flags being set.
The third to last page, aka page 255-511-509, is the usyscall page used in the last task.

1
kernel/defs.h
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@ -173,6 +173,7 @@ uint64 walkaddr(pagetable_t, uint64);
int copyout(pagetable_t, uint64, char *, uint64);
int copyin(pagetable_t, char *, uint64, uint64);
int copyinstr(pagetable_t, char *, uint64, uint64);
void vmprint(pagetable_t pagetable);
// plic.c
void plicinit(void);

1
kernel/exec.c
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@ -128,6 +128,7 @@ exec(char *path, char **argv)
p->trapframe->sp = sp; // initial stack pointer
proc_freepagetable(oldpagetable, oldsz);
if (p->pid == 1) vmprint(p->pagetable);
return argc; // this ends up in a0, the first argument to main(argc, argv)
bad:

25
kernel/vm.c
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@ -437,3 +437,28 @@ copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max)
return -1;
}
}
static void _vmprint(pagetable_t pagetable, int depth)
{
if (depth == 3)
return;
for (int i = 0; i < 512; ++i)
{
pte_t pte = pagetable[i];
if (pte & PTE_V)
{
for (int _i = 0; _i < depth; ++_i)
{
printf(".. ");
}
printf("..%d: pte %p pa %p\n", i, pte, PTE2PA(pte));
_vmprint((pagetable_t)PTE2PA(pte), depth + 1);
}
}
}
void vmprint(pagetable_t pagetable)
{
printf("page table %p\n", pagetable);
_vmprint(pagetable, 0);
}