学习xenstore

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XenStore是一个类似于数据库的文件系统, 包含了domain间的共享信息. 有domain配置和状态信息.XenStore 提供了一种发现设备信息的简便方法. 它作为数据库在 /var/lib/xenstore/tdb, 在用户空间的 daemon 称为 "xenstored".这个逻辑文件树有三个主要的路径:

/vm - /vm/uuid 存储配置信息,例如虚拟CPU数和内存分配数.

/local/domain - 包含了正在运行的domain信息, 由domid标识.

/tool - 存储多种工具信息.

应用程序向这个数据库的 key 写信息, 配置驱动; 驱动在key上设置watch并对改变做出回应.

详细介绍请参考: http://wiki.xensource.com/xenwiki/XenStoreReference

访问xenstore

在guest kernel启动时xenstore经过start_info页而被访问到, 作为共享页的机器页帧号和event channel使用.Dom0工具使用UNIX domain socket /var/run/xenstored/socket 或 /var/run/xenstoed/socket_ro; Dom0工具使用proce接口/proc/xen/xenbus, 内核代码使用xenbus API.

命令

xenstore-read, xenstore-exists, xenstore-list, xenstor-, xsls, 用于和xenstored daemon数据库通信.

e.g. - #xenstore-list /local/domain/0

Domain 标识

1) 通用唯一标识符(UUID)是标识domain的数字, 即使guest迁移也保持相同.

2) domain 标识(DOMID) 标识一个正在运行的实例, 当guest 迁移到另一台机器后DOMID改变.

什么是Xenbus

Xenbus是Xenstore的一个接口, 它也是在Xenstore之上写的设备驱动的连接协议. Xenbus是一个虚拟设备的mgmt bus. Xen PV "backend" 设备出现在xenbus上, 而且能被DomU使用PV "fronted"设备驱动访问到. 这些虚拟后端设备的状态由xenbus管理. 这些状态被存储到xenstore, 并且一直被Dom0和DomU的xenbus驱动程序监测.XenStore 指供了一个domain如何将前端设备与后端提供的服务联系起来的方法, (实际这部分是由XenBus承载的, 一种建立在XenStore顶层的协议). 后端与前端通信使用共享event channel 和 ring buffer通信. 所有规范的xen虚拟设备驱动在初始化时向XenBus注册自己. 这是通过传递xenbus_driver 结构体给

xenbus_register_devide()函数.

代码

以下代码创建两个可执行的, xenstore-read 和 xenstore-write 程序.

在一个Dom中运行xenstore-read - 从 /loca/doman/X/memory/target 读取Domx的memory/target值.依赖于你运行这个程序所在的Dom.

在一个Dom中运行xenstore-wirte - 写一个memory/target值到这个程序所运行的Dom.

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// Example taken from wiki.xensource.com/xenwiki/XenStoreReference and some code added
//
// Compiled using
// gcc -g xenstore.c -DREAD=1 -lxenstore -o xenstore-read
// gcc -g xenstore.c -DWRITE=1 -lxenstore -o xenstore-write
//
// To change permissions on /local/domain/0, use
// xenstore-chod -r /local/domain/0 b - for r and w
// xenstore-chod -r /local/domain/0 w - for w
//
#include <sys/types.h>
#include <xs.h>
#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include <sys/stat.h> // for lstat()
#include <stdlib.h> // for exit()
#define DOM0 0
#define DOMU 1
int dom = 999;
// This function checks if the app is in domU or dom0. The /proc/xen/capabilities
// contains the string "control_d" if the app is in a dom0
//
// /sys/hypervisor/type can be checked too, to see if we are running on a xen
// hyervisor
check_dom()
{
FILE *file = NULL;
char *filePath;
struct stat linkAttrs;
char buf[100] = "";
filePath = "/proc/xen/capabilities";
if ((lstat(filePath, &linkAttrs)) != 0) {
perror("lstat");
return;
}
if ((file = fopen("/proc/xen/capabilities", "r")) == NULL) {
perror("fopen");
return;
}
if (!fgets(buf, sizeof(buf), file)) {
// we are in DomU, since the buffer is empty
printf("/n Surely in DOMU, since capabilities is empty");
dom = DOMU;
return;
} else {
// we are probably in Dom0
printf("/n Probably in DOM0, since capabilities has some data");
dom = DOM0;
}
// the following is in case they change the capabilities to have some
// data in a DomU
if ((strstr(buf, "control_d")) == NULL) {
// we are in DomU
printf("/n We are in DOMU");
dom = DOMU;
} else {
// we are in Dom0
printf("/n We are in DOM0");
dom = DOM0;
}
return;
}
// xs_write is written outside a transaction. Else it does not work for me. Pass
// a XBT_NULL as a 2nd param to xs_write().
write_to_xenstore (struct xs_handle *xs, char* path) {
char somedata[8] = {"2200000"};
xs_transaction_t trans;
bool err;
printf("/nWriting data %s of len %d to %s",
somedata, strlen(somedata), path);
// xs_write() -> xs_talkv()
// -> xs_write_all() -> write() - sys_write()
// -> xs_read_reply()
err = xs_write(xs, XBT_NULL, path, &somedata[0],
strlen(somedata));
if (!err) {
printf("/n Could'nt write var in xenstore");
}
xs_daemon_close(xs);
free(path);
exit(0);
}
int main(int argc, char *argv[]) {
struct xs_handle *xs; // handle to xenstore
xs_transaction_t trans;
char *path;
int fd;
fd_set set;
bool err;
struct timeval tv = {.tv_sec = 0, .tv_usec = 0};
char **vec;
unsigned int num;
char *buf;
char **buf2;
unsigned int len, i;
unsigned int domid;
// check if we are in Dom0 or DomU
check_dom();
// Connect to XenStore. From Dom0 we call xs_daemon_open,
// and from DomU, we call xs_domain_open().
// xs_daemon_open() -> get_handle(xs_daemon_socket()).
// xs_daemon_socket()->xs_daemon_path() which returns
// XENSTORED_PATH/socket. A get_handle() on the above path generates
// a socket (PF_UNIX, SOCK_STREAM, 0) and connects to above socket.
// For domU, connect to XenStore via xs_domain_open().
// xs_domain_dev() returns path to /proc/xen/xenbus, and a
// get_handle() on this returns a 'fd' to this file.
if (dom == DOMU) {
xs = xs_domain_open(); // DomU
if (xs == NULL) error();
buf = xs_read(xs, XBT_NULL, "domid", &len);
if (!buf) {
printf("/n Could not read domid");
return;
}
domid = atoi(buf);
printf("/n Retrieved Dom ID = %d/n", domid);
} else {
xs = xs_daemon_open(); // Dom0
if (xs == NULL) error();
trans = xs_transaction_start(xs);
if (trans == 0) {
printf("/n Could not start xaction with XS"); return;
}
// Get contents of a directory. Need to call free after use,
// since the API mallocs memory
buf2 = xs_directory(xs, trans, "/local/domain", &len);
if (!buf2) {
printf("/n Could not read XS dir /local/domain"); return;
}
xs_transaction_end(xs, trans, true);
if (trans == 0) {
printf("/n Could not end xaction with XS"); return;
}
printf("/n Len of Dir /local/domain is %d", len);
printf("/n Dir Contents: ");
for (i=0; i<len;i++) { printf("%s ", buf2[i]); }
if (len == 1) {
// we have only 1 Dom running, i.e. Dom0
domid = atoi(buf2[0]);
} else if (len == 2) {
domid = atoi(buf2[1]);
}
// Set domid to 0, since Dom0 is always id 0
domid = 0;
printf("/n Setting Dom ID = %d/n", domid);
}
// Get the local Domain path in xenstore
path = xs_get_domain_path(xs, domid);
if (path == NULL) {
printf("/n Dom Path in Xenstore not found");
error(); return;
}
// xs_directory has an implicit root at /local/domain/<domid> in DomU
// and thus need to just pass "memory/target" if we are running this
// from a DomU to read the directory's contents
if (dom == DOM0) {
path = realloc(path, strlen(path) + strlen("/memory/target") + 1);
if (path == NULL) {
error(); return;
}
strcat(path, "/memory/target");
} else {
strcpy(path, "memory/target");
}
printf("/nPath = %s", path);
// If we are doing a write to the xenstore, branch off here and
// then exit from the program. Else for read, cary on, and do a
// select() to wait for a change in watched values
#ifdef WRITE
write_to_xenstore(xs, path);
#endif
// Create a watch on the path
err = xs_watch(xs, path, "mytoken");
if (err == 0) {
printf("/n Error in setting watch on mytoken in %s", path);
error(); return;
}
// Watches are notified via a File Descriptor. We can POLL on this.
fd = xs_fileno(xs);
while (1) {
FD_ZERO(&set);
FD_SET(fd, &set);
printf("!-");
fflush(stdout);
struct timeval tv = {.tv_sec = 5, .tv_usec = 0};
if (select(fd+1, &set, NULL, NULL, &tv) > 0
&& FD_ISSET(fd, &set)) {
printf("@");
// This blocks is nothing is pending. Returns an array
// containing path and token. Use Xs_WATCH_* to
// access these elements. Call free after use.
vec = xs_read_watch(xs, &num);
if (!vec) {
printf("Error on watch firing");
error(); return;
}
// In our example code, the following will print out
// /local/domain/0/memory/target|mytoken
printf("/nvec contents: %s|%s/n", vec[XS_WATCH_PATH],
vec[XS_WATCH_TOKEN]);
// Prepare a transacation to do a read
trans = xs_transaction_start(xs);
if (trans == 0) {
printf("/n Could'nt start xaction xenstore");
return;
}
buf = xs_read(xs, trans, vec[XS_WATCH_PATH], &len);
if (!buf) {
printf("/n Could'nt read watch var in vec");
return;
}
xs_transaction_end(xs, trans, true);
if (trans == 0) {
printf("/n Could not end xaction xenstore");
return;
}
if (buf) {
printf("/n buflen: %d, buf: %s", len, buf);
}
} // end of select
} // end while(1)
printf("/n");
// cleanup
close(fd);
xs_daemon_close(xs);
free(path);
return;
}

参考:

1] The Definitive Guide to the Xen Hypervisor David Chisnall

2] http://www.cs.uic.edu/~spopuri/minios.html

3] http://wiki.xensource.com/xenwiki/XenStoreReference

4] http://wiki.xensource.com/xenwiki/XenBus

5] http://lists.xensource.com/archives/html/xen-devel/2005-12/msg00151.html

6] http://wiki.xensource.com/xenwiki/XenBus

7] http://knol.google.com/k/learning-xenstore