Speed up system calls 加速系统调用

有些操作系统会通过在用户空间和内核空间之间共享一些内存来加速系统调用。该实验题目要求在内核中保存一个映射到用户内存空间的结构体，该结构体中保存当前进程的pid以加速pid的获取。

// Look in the process table for an UNUSED proc.
// If found, initialize state required to run in the kernel,
// and return with p->lock held.
// If there are no free procs, or a memory allocation fails, return 0.
static struct proc*
allocproc(void)
{struct proc *p;for(p = proc; p < &proc[NPROC]; p++) {acquire(&p->lock);if(p->state == UNUSED) {goto found;} else {release(&p->lock);}}return 0;found:p->pid = allocpid();p->state = USED;// Allocate a trapframe page.if((p->trapframe = (struct trapframe *)kalloc()) == 0){freeproc(p);release(&p->lock);return 0;}// Allocate a usyscall page.if((p->usyscall = (struct usyscall *)kalloc()) == 0){freeproc(p);release(&p->lock);return 0;}p->usyscall->pid = p->pid;// An empty user page table.p->pagetable = proc_pagetable(p);if(p->pagetable == 0){freeproc(p);release(&p->lock);return 0;}// Set up new context to start executing at forkret,// which returns to user space.memset(&p->context, 0, sizeof(p->context));p->context.ra = (uint64)forkret;p->context.sp = p->kstack + PGSIZE;return p;
}// free a proc structure and the data hanging from it,
// including user pages.
// p->lock must be held.
static void
freeproc(struct proc *p)
{if(p->usyscall) {kfree((void*)p->usyscall);}p->usyscall = 0;if(p->trapframe)kfree((void*)p->trapframe);p->trapframe = 0;if(p->pagetable)proc_freepagetable(p->pagetable, p->sz);p->pagetable = 0;p->sz = 0;p->pid = 0;p->parent = 0;p->name[0] = 0;p->chan = 0;p->killed = 0;p->xstate = 0;p->state = UNUSED;
}// Create a user page table for a given process,
// with no user memory, but with trampoline pages.
pagetable_t
proc_pagetable(struct proc *p)
{pagetable_t pagetable;// An empty page table.pagetable = uvmcreate();if(pagetable == 0)return 0;// map the trampoline code (for system call return)// at the highest user virtual address.// only the supervisor uses it, on the way// to/from user space, so not PTE_U.if(mappages(pagetable, TRAMPOLINE, PGSIZE,(uint64)trampoline, PTE_R | PTE_X) < 0){uvmfree(pagetable, 0);return 0;}// map the trapframe just below TRAMPOLINE, for trampoline.S.if(mappages(pagetable, TRAPFRAME, PGSIZE,(uint64)(p->trapframe), PTE_R | PTE_W) < 0){uvmunmap(pagetable, TRAMPOLINE, 1, 0);uvmfree(pagetable, 0);return 0;}// map the usyscall below trapframeif(mappages(pagetable, USYSCALL, PGSIZE,(uint64)(p->usyscall), PTE_R | PTE_U) < 0){uvmunmap(pagetable, TRAMPOLINE, 1, 0);uvmunmap(pagetable, TRAPFRAME, 1, 0);uvmfree(pagetable, 0);return 0;}return pagetable;
}// Free a process's page table, and free the
// physical memory it refers to.
void
proc_freepagetable(pagetable_t pagetable, uint64 sz)
{uvmunmap(pagetable, TRAMPOLINE, 1, 0);uvmunmap(pagetable, TRAPFRAME, 1, 0);uvmunmap(pagetable, USYSCALL, 1, 0);uvmfree(pagetable, sz);
}

打印页表 Print a page table

// 递归打印页表
void vmprint_search(int depth, pte_t pte, int index) {for(int i = 0; i < depth; i++) {printf(".. ");}pagetable_t pt = (pagetable_t)PTE2PA(pte);printf("%d: pte %p pa %p\n", index, pte, pt);if ((pte & (PTE_R|PTE_W|PTE_X)) != 0) {return;}for (int i = 0; i < 512; i++) {pte = pt[i];if((pte & PTE_V) && (pte & (PTE_R|PTE_W|PTE_X)) == 0) {vmprint_search(depth + 1, pte, i);} else if (pte & PTE_V) {vmprint_search(depth + 1, pte, i);}}
}// print vm.
void vmprint(pagetable_t pagetable)
{printf("page table %p\n", pagetable);// there are 2^9 = 512 PTEs in a page table.for(int i = 0; i < 512; i++){pte_t pte = pagetable[i];if((pte & PTE_V) && (pte & (PTE_R|PTE_W|PTE_X)) == 0){// this PTE points to a lower-level page table.vmprint_search(1, pte, i);}}
}

Detecting which pages have been accessed

Your job is to implement pgaccess(), a system call that reports which pages have been accessed. The system call takes three arguments. First, it takes the starting virtual address of the first user page to check. Second, it takes the number of pages to check. Finally, it takes a user address to a buffer to store the results into a bitmask (a datastructure that uses one bit per page and where the first page corresponds to the least significant bit). You will receive full credit for this part of the lab if the pgaccess test case passes when running pgtbltest.

int
sys_pgaccess(void)
{// lab pgtbl: your code here.uint64 addr;int page_num;uint64 mask_addr;if(argaddr(0, &addr) < 0)return -1;if(argint(1, &page_num) < 0)return -1;if(argaddr(2, &mask_addr) < 0)return -1;struct proc *the_proc = myproc();pagetable_t pt = the_proc->pagetable;uint32 mask = 0;for(int i = 0; i < page_num; i++) {pte_t *pte = walk(pt, addr + i * PGSIZE, 0);if ((*pte & PTE_A)) {mask = mask | (1 << i);*pte = *pte & (~PTE_A);}}copyout(pt, mask_addr, (char*)&mask, sizeof(uint32));return 0;
}

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6.S081 Lab3 page tables 页表

Speed up system calls 加速系统调用

打印页表 Print a page table

Detecting which pages have been accessed

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