MIT 6.S081 - LAB 3

Speed up system calls

Some operating systems (e.g., Linux) speed up certain system calls by sharing data in a read-only region between userspace and the kernel. This eliminates the need for kernel crossings when performing these system calls. To help you learn how to insert mappings into a page table, your first task is to implement this optimization for the getpid() system call in xv6.

When each process is created, map one read-only page at USYSCALL (a VA defined in memlayout.h). At the start of this page, store a struct usyscall (also defined in memlayout.h), and initialize it to store the PID of the current process. For this lab, ugetpid() has been provided on the userspace side and will automatically use the USYSCALL mapping. You will receive full credit for this part of the lab if the ugetpid test case passes when running pgtbltest.
Some hints:

• You can perform the mapping in proc_pagetable() in kernel/proc.c.
• Choose permission bits that allow userspace to only read the page.
• You may find that mappages() is a useful utility.
• Don't forget to allocate and initialize the page in allocproc().
• Make sure to free the page in freeproc().

Solution code

coding...