日期：2015年11月6日

libvirt笔记 (3) —— 得到virtualization host的能力信息

getCapabilities方法得到一个字符串，用来描述virtualization host的能力，以及能创建什么样的Guest OS。请看下面代码：

#!/usr/bin/python

from __future__ import print_function
import sys
import libvirt

conn = libvirt.open('xen:///')
if conn == None:
    print('Failed to open connection to xen:///', file=sys.stderr)
    exit(1)

caps = conn.getCapabilities() # caps will be a string of XML
print('Capabilities:\n'+caps)

conn.close()
exit(0)

执行如下：

Capabilities:
<capabilities>

  <host>
    <cpu>
      <arch>x86_64</arch>
      <features>
        <pae/>
      </features>
    </cpu>
    <power_management/>
    <migration_features>
      <live/>
    </migration_features>
    <topology>
      <cells num='1'>
        <cell id='0'>
          <memory unit='KiB'>1048512</memory>
          <cpus num='0'>
          </cpus>
        </cell>
      </cells>
    </topology>
  </host>

  <guest>
    <os_type>xen</os_type>
    <arch name='x86_64'>
      <wordsize>64</wordsize>
      <emulator>/usr/lib/xen/bin/qemu-system-i386</emulator>
      <machine>xenpv</machine>
      <domain type='xen'/>
    </arch>
  </guest>

  <guest>
    <os_type>xen</os_type>
    <arch name='i686'>
      <wordsize>32</wordsize>
      <emulator>/usr/lib/xen/bin/qemu-system-i386</emulator>
      <machine>xenpv</machine>
      <domain type='xen'/>
    </arch>
    <features>
      <pae/>
    </features>
  </guest>

</capabilities>

参考资料：
Capability information。

 

SystemTap 笔记 （4）—— timer event

timer event会周期性执行handler。举个例子：

# stap -e 'probe timer.s(1) { printf("Hello world!\n");}'
Hello world!
Hello world!
Hello world!
Hello world!

上面脚本每隔1秒打印一次Hello world!。

timer event定义如下：

timer.ms(milliseconds)
timer.us(microseconds)
timer.ns(nanoseconds)
timer.hz(hertz)
timer.jiffies(jiffies)

另外，还有一种randomize表示方式（参考自这里）：

timer.jiffies(N).randomize(M)

The probe handler is run every N jiffies (a kernel-defined unit of time, typically between 1 and 60 ms). If the “randomize” component is given, a linearly distributed random value in the range [-M..+M] is added to N every time the handler is run. N is restricted to a reasonable range (1 to around a million), and M is restricted to be smaller than N.

Alternatively, intervals may be specified in units of time. There are two probe point variants similar to the jiffies timer:

timer.ms(N)

timer.ms(N).randomize(M)

Here, N and M are specified in milliseconds, but the full options for units are seconds (s/sec), milliseconds (ms/msec), microseconds (us/usec), nanoseconds (ns/nsec), and hertz (hz). Randomization is not supported for hertz timers.

最后结合一个例子看一下如何使用timer event（选自这里）：

global count_jiffies, count_ms
probe timer.jiffies(100) { count_jiffies ++ }
probe timer.ms(100) { count_ms ++ }
probe timer.ms(12345)
{
  hz=(1000*count_jiffies) / count_ms
  printf ("jiffies:ms ratio %d:%d => CONFIG_HZ=%d\n",
    count_jiffies, count_ms, hz)
  exit ()
}

首先要知道，每秒发生HZ次jiffies。

其次，每发生100次jiffies，count_jiffies计数加1，所以脚本退出时，一共发生100 * count_jiffies次HZ。一共经历了count_ms / 10秒。

最后计算CONFIG_HZ：(100 * count_jiffies) / (count_ms / 10) = (1000 * count_jiffies) / count_ms。

 

Linux kernel 笔记 （39）——”THIS_MODULE”

THIS_MODULE是一个macro，定义在<linux/module.h>中：

#ifdef MODULE
#define MODULE_GENERIC_TABLE(gtype,name)            \
extern const struct gtype##_id __mod_##gtype##_table        \
  __attribute__ ((unused, alias(__stringify(name))))

extern struct module __this_module;
#define THIS_MODULE (&__this_module)
#else  /* !MODULE */
#define MODULE_GENERIC_TABLE(gtype,name)
#define THIS_MODULE ((struct module *)0)
#endif

THIS_MODULE即是__this_module这个变量的地址。__this_module会指向这个模块起始的地址空间，恰好是struct module变量定义的位置。

file_operations结构体的第一个成员是struct module类型的指针，定义在<linux/fs.h>中：

struct file_operations {
    struct module *owner;
    ......
}

LDD对其的解释：

struct module *owner

The first file_operations field is not an operation at all; it is a pointer to the module that “owns” the structure. This field is used to prevent the module from being unloaded while its operations are in use. Almost all the time, it is simply initialized to THIS_MODULE , a macro defined in <linux/module.h>.

owner指向绑定file_operations的模块。在大多时候，只需把THIS_MODULE赋给它即可。

参考资料：
Where is the memory allocation of “_thismodule” variable?；
深入淺出 insmod, #1。

 

Linux kernel 笔记 （38）——”__user”修饰符

在kernel代码中，有时会看到函数声明中有的参数带有__user修饰符：

ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

LDD给出的解释：

This annotation is a form of documentation, noting that a pointer is a user-space address that cannot be directly dereferenced. For normal compilation, __user has no effect, but it can be used by external checking software to find misuse of user-space addresses.

__user表明参数是一个用户空间的指针，不能在kernel代码中直接访问。也方便其它工具对代码进行检查。