KVM虚拟机IO处理过程(一) ----Guest VM I/O 处理过程

    虚拟化技术主要包含三部分内容:CPU虚拟化,内存虚拟化,设备虚拟化.本系列文章主要描述磁盘设备的虚拟化过程,包含了一个读操作的I/O请求如何从Guest Vm到其最终被处理的整个过程.本系列文章中引用到的linux内核代码版本为3.7.10,使用的虚拟化平台是KVM,qemu的版本是1.6.1.

    用户程序想要访问IO设备需要调用操作系统提供的接口,即系统调用.当在用户程序中调用一个read操作时,系统先保存好read操作的参数,然后调用int 80命令(也可能是sysenter)进入内核空间,在内核空间中,读操作的逻辑由sys_read函数实现.

    在讲sys_read的实现过程之前,我们先来看看read操作在内核空间需要经历的层次结构.从图中可以看出,read操作首先经过虚拟文件系统曾(vfs), 接下来是具体的文件系统层,Page cache层,通用块层(generic block layer),I/O调度层(I/O scheduler layer),块设备驱动层(block device driver layer),最后是块物理设备层(block device layer).

• 虚拟文件系统层:该层屏蔽了下层的具体操作,为上层提供统一的接口,如vfs_read,vfs_write等.vfs_read,vfs_write通过调用下层具体文件系统的接口来实现相应的功能.
• 具体文件系统层:该层针对每一类文件系统都有相应的操作和实现了,包含了具体文件系统的处理逻辑.
• page cache层:该层缓存了从块设备中获取的数据.引入该层的目的是避免频繁的块设备访问,如果在page cache中已经缓存了I/O请求的数据,则可以将数据直接返回,无需访问块设备.
• 通过块层:接收上层的I/O请求,并最终发出I/O请求.该层向上层屏蔽了下层设备的特性.
• I/O调度层:   接收通用块层发出的 IO 请求，缓存请求并试图合并相邻的请求（如果这两个请求的数据在磁盘上是相邻的）。并根据设置好的调度算法，回调驱动层提供的请求处理函数，以处理具体的 IO 请求
• 块设备驱动层:从上层取出请求,并根据参数,操作具体的设备.
• 块设备层:真正的物理设备.

    了解了内核层次的结构,让我们来看一下read操作的代码实现.

     sys_read函数声明在include/linux/syscalls.h文件中,

asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);

     

      其函数实现在fs/read_write.c文件中:

SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count){struct fd f = fdget(fd);ssize_t ret = -EBADF;if (f.file) {loff_t pos = file_pos_read(f.file);ret = vfs_read(f.file, buf, count, &pos); //调用vfs layer中的read操作file_pos_write(f.file, pos);//设置当前文件的位置fdput(f);}return ret;}

    vfs_read函数属于vfs layer,定义在fs/read_write.c, 其主要功能是调用具体文件系统中对应的read操作,如果具体文件系统没有提供read操作,则使用默认的do_sync_read函数.

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos){ssize_t ret;if (!(file->f_mode & FMODE_READ))return -EBADF;if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))return -EINVAL;if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))return -EFAULT;ret = rw_verify_area(READ, file, pos, count);if (ret >= 0) {count = ret;if (file->f_op->read) {ret = file->f_op->read(file, buf, count, pos); //该函数由具体的文件系统指定} elseret = do_sync_read(file, buf, count, pos);  //内核默认的读文件操作if (ret > 0) {fsnotify_access(file);add_rchar(current, ret);}inc_syscr(current);}return ret;}

    file->f_op的类型为struct file_operations, 该类型定义了一系列涉及文件操作的函数指针,针对不同的文件系统,这些函数指针指向不同的实现.以ext4 文件系统为例子,该数据结构的初始化在fs/ext4/file.c,从该初始化可以知道,ext4的read操作调用了内核自带的do_sync_read()函数

const struct file_operations ext4_file_operations = {.llseek= ext4_llseek,.read= do_sync_read,.write= do_sync_write,.aio_read= generic_file_aio_read,.aio_write= ext4_file_write,.unlocked_ioctl = ext4_ioctl,#ifdef CONFIG_COMPAT.compat_ioctl= ext4_compat_ioctl,#endif.mmap= ext4_file_mmap,.open= ext4_file_open,.release= ext4_release_file,.fsync= ext4_sync_file,.splice_read= generic_file_splice_read,.splice_write= generic_file_splice_write,.fallocate= ext4_fallocate,};

    do_sync_read()函数定义fs/read_write.c中,

ssize_t do_sync_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos){struct iovec iov = { .iov_base = buf, .iov_len = len };struct kiocb kiocb;ssize_t ret;init_sync_kiocb(&kiocb, filp);//初始化kiocp,描述符kiocb是用来记录I/O操作的完成状态kiocb.ki_pos = *ppos;kiocb.ki_left = len;kiocb.ki_nbytes = len;for (;;) {ret = filp->f_op->aio_read(&kiocb, &iov, 1, kiocb.ki_pos);//调用真正做读操作的函数,ext4文件系统在fs/ext4/file.c中配置if (ret != -EIOCBRETRY)break;wait_on_retry_sync_kiocb(&kiocb);}if (-EIOCBQUEUED == ret)ret = wait_on_sync_kiocb(&kiocb);*ppos = kiocb.ki_pos;return ret;}

    在ext4文件系统中filp->f_op->aio_read函数指针只想generic_file_aio_read, 该函数定义于mm/filemap.c文件中,该函数有两个执行路径,如果是以O_DIRECT方式打开文件,则读操作跳过page cache直接去读取磁盘,否则调用do_generic_sync_read函数尝试从page cache中获取所需的数据.

ssize_tgeneric_file_aio_read(struct kiocb *iocb, const struct iovec *iov,unsigned long nr_segs, loff_t pos){struct file *filp = iocb->ki_filp;ssize_t retval;unsigned long seg = 0;size_t count;loff_t *ppos = &iocb->ki_pos;count = 0;retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);if (retval)return retval;/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */if (filp->f_flags & O_DIRECT) {loff_t size;struct address_space *mapping;struct inode *inode;struct timex txc;do_gettimeofday(&(txc.time));mapping = filp->f_mapping;inode = mapping->host;if (!count)goto out; /* skip atime */size = i_size_read(inode);if (pos < size) {retval = filemap_write_and_wait_range(mapping, pos,pos + iov_length(iov, nr_segs) - 1);if (!retval) {retval = mapping->a_ops->direct_IO(READ, iocb,iov, pos, nr_segs);}if (retval > 0) {*ppos = pos + retval;count -= retval;}/* * Btrfs can have a short DIO read if we encounter * compressed extents, so if there was an error, or if * we've already read everything we wanted to, or if * there was a short read because we hit EOF, go ahead * and return.  Otherwise fallthrough to buffered io for * the rest of the read. */if (retval < 0 || !count || *ppos >= size) {file_accessed(filp);goto out;}}}count = retval;for (seg = 0; seg < nr_segs; seg++) {read_descriptor_t desc;loff_t offset = 0;/* * If we did a short DIO read we need to skip the section of the * iov that we've already read data into. */if (count) {if (count > iov[seg].iov_len) {count -= iov[seg].iov_len;continue;}offset = count;count = 0;}desc.written = 0;desc.arg.buf = iov[seg].iov_base + offset;desc.count = iov[seg].iov_len - offset;if (desc.count == 0)continue;desc.error = 0;do_generic_file_read(filp, ppos, &desc, file_read_actor);retval += desc.written;if (desc.error) {retval = retval ?: desc.error;break;}if (desc.count > 0)break;}out:return retval;}

    do_generic_file_read定义在mm/filemap.c文件中,该函数调用page cache层中相关的函数.如果所需数据存在与page cache中,并且数据不是dirty的,则从page cache中直接获取数据返回.如果数据在page cache中不存在,或者数据是dirty的,则page cache会引发读磁盘的操作.该函数的读磁盘并不是简单的只读取所需数据的所在的block,而是会有一定的预读机制来提高cache的命中率,减少磁盘访问的次数. 

    page cache层中真正读磁盘的操作为readpage系列,readpage系列函数具体指向的函数实现在fs/ext4/inode.c文件中定义,该文件中有很多个struct address_space_operation对象来对应与不同日志机制,我们选择linux默认的ordered模式的日志机制来描述I/O的整个流程, ordered模式对应的readpage系列函数如下所示.

static const struct address_space_operations ext4_ordered_aops = {.readpage= ext4_readpage,.readpages= ext4_readpages,.writepage= ext4_writepage,.write_begin= ext4_write_begin,.write_end= ext4_ordered_write_end,.bmap= ext4_bmap,.invalidatepage= ext4_invalidatepage,.releasepage= ext4_releasepage,.direct_IO= ext4_direct_IO,.migratepage= buffer_migrate_page,.is_partially_uptodate  = block_is_partially_uptodate,.error_remove_page= generic_error_remove_page,};

    为简化流程,我们选取最简单的ext4_readpage函数来说明,该函数实现位于fs/ext4/inode.c中,函数很简单,只是调用了mpage_readpage函数.mpage_readpage位于fs/mpage.c文件中,该函数生成一个IO请求,并提交给Generic block layer.

int mpage_readpage(struct page *page, get_block_t get_block){struct bio *bio = NULL;sector_t last_block_in_bio = 0;struct buffer_head map_bh;unsigned long first_logical_block = 0;map_bh.b_state = 0;map_bh.b_size = 0;bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,&map_bh, &first_logical_block, get_block);if (bio)mpage_bio_submit(READ, bio);return 0;}

    Generic block layer会将该请求分发到具体设备的IO队列中,由I/O Scheduler去调用具体的driver接口获取所需的数据.

    至此,在Guest vm中整个I/O的流程已经介绍完了,后续的文章会介绍I/O操作如何从Guest vm跳转到kvm及如何在qemu中模拟I/O设备.

参考资料:

1. read系统调用剖析:http://www.ibm.com/developerworks/cn/linux/l-cn-read/