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之前打算整理一下在Guest VM, KVM, QEMU中IO处理的整个流程,通过查阅资料和阅读源码,已经大致知道IO在Guest KVM中的处理流程.当想要整理IO在KVM和QEMU中的处理时,发现很难理清楚QEMU和KVM之间的跳转和交互的过程,于是促使自己去了解QEMU和KVM启动的过程.(本文展示的代码中,qemu版本为1.6.0, linux内核版本为3.7.10)
为了介绍qemu和kvm的交互过程,我首先介绍一下kvm给用户提供的接口.kvm是一个内核模块,它实现了一个/dev/kvm的字符设备来与用户进行交互,通过调用一系列ioctl函数可以实现qemu和kvm之间的切换.当要创建一个新的虚拟机时,首先打开/dev/kvm设备,在其上调用ioctl函数:
- system_fd = open("/dev/kvm", ORDWR);
- vm_fd = ioctl(system_fd, KVM_CREATE_VM, 0);
创建完虚拟机之后,需要在该虚拟机上面创建vcpu,调用的接口也是ioctl,只是此时对应的fd为创建虚拟机时返回的fd.
- vcpu_fd = ioctl(vm_fd, VM_CREATE_VCPU, 0)
创建完vcpu后,可以在该vcpu上面调用ioctl函数进入guest vm.
- ret = ioctl(vcpu_fd, KVM_RUN, 0);
qemu作为一个user mode的程序,其入口为main函数,该main函数定义在vl.c文件中.main函数比较长,其中跟KVM初始化相关的主要有两个函数:configure_accelerator()和machine->init(&args). cofigure_accelerator()函数选择运用哪一种虚拟化方案,其应用到的数据结构为accel_list,会调用accel_list[i].init函数.accel_list的初始化如下所示,当使用kvm虚拟化解决方案时,accel_list[i].init对应的函数即为kvm_init.
- static struct {
- const char *opt_name;
- const char *name;
- int (*available)(void);
- int (*init)(void);
- bool *allowed;
- } accel_list[] = {
- { "tcg", "tcg", tcg_available, tcg_init, &tcg_allowed },
- { "xen", "Xen", xen_available, xen_init, &xen_allowed },
- { "kvm", "KVM", kvm_available, kvm_init, &kvm_allowed },
- { "qtest", "QTest", qtest_available, qtest_init, &qtest_allowed },
- };
machine->init(&arg)函数主要初始化硬件设备,并且调用qemu_init_vcpu为每一个vcpu创建一个线程,线程执行的函数为qemu_kvm_cpu_thread_fn.从qemu main到qemu_init_vcpu之间函数调用关系涉及到一些函数指针的赋值源码比较难于读懂,以下是使用gdb调试打出其调用关系.
- #0 qemu_init_vcpu (cpu=0x55555681ea90) at /home/dashu/kvm/qemu/qemu-dev-zwu/cpus.c:1084
- #1 0x0000555555909f1e in x86_cpu_realizefn (dev=0x55555681ea90, errp=0x7fffffffd8f8) at /home/dashu/kvm/qemu/qemu-dev-zwu/target-i386/cpu.c:2399
- #2 0x00005555556c768a in device_set_realized (obj=0x55555681ea90, value=true, err=0x7fffffffda88) at hw/core/qdev.c:699
- #3 0x000055555580b93f in property_set_bool (obj=0x55555681ea90, v=0x5555565bab20, opaque=0x5555565375a0, name=0x555555a01f88 "realized", errp=0x7fffffffda88) at qom/object.c:1300
- #4 0x000055555580a484 in object_property_set (obj=0x55555681ea90, v=0x5555565bab20, name=0x555555a01f88 "realized", errp=0x7fffffffda88) at qom/object.c:788
- #5 0x000055555580bbea in object_property_set_qobject (obj=0x55555681ea90, value=0x555556403e40, name=0x555555a01f88 "realized", errp=0x7fffffffda88) at qom/qom-qobject.c:24
- #6 0x000055555580a770 in object_property_set_bool (obj=0x55555681ea90, value=true, name=0x555555a01f88 "realized", errp=0x7fffffffda88) at qom/object.c:851
- #7 0x00005555558a7de0 in pc_new_cpu (cpu_model=0x555555a0200b "qemu64", apic_id=0, icc_bridge=0x55555655b2c0, errp=0x7fffffffdac8) at /home/dashu/kvm/qemu/qemu-dev-zwu/hw/i386/pc.c:922
- #8 0x00005555558a7fed in pc_cpus_init (cpu_model=0x555555a0200b "qemu64", icc_bridge=0x55555655b2c0) at /home/dashu/kvm/qemu/qemu-dev-zwu/hw/i386/pc.c:978
- #9 0x00005555558a923b in pc_init1 (system_memory=0x5555562a7240, system_io=0x5555562a7f60, ram_size=1073741824, boot_device=0x555555a0248a "cad", kernel_filename=0x0, kernel_cmdline=0x5555559f85be "",
- initrd_filename=0x0, cpu_model=0x0, pci_enabled=1, kvmclock_enabled=1) at /home/dashu/kvm/qemu/qemu-dev-zwu/hw/i386/pc_piix.c:105
- #10 0x00005555558a9a36 in pc_init_pci (args=0x7fffffffdf10) at /home/dashu/kvm/qemu/qemu-dev-zwu/hw/i386/pc_piix.c:245
- #11 0x00005555558a9a7f in pc_init_pci_1_6 (args=0x7fffffffdf10) at /home/dashu/kvm/qemu/qemu-dev-zwu/hw/i386/pc_piix.c:255
- #12 0x00005555558584fe in main (argc=10, argv=0x7fffffffe148, envp=0x7fffffffe1a0) at vl.c:4317
qemu_kvm_cpu_thread_fn函数创建vcpu,然后调用kvm_cpu_exec函数.kvm_cpu_exec函数调用ioctl进入kvm并最终进入guest vm.
以上即为qemu调用kvm的接口初始化kvm的过程.后续我会整理出IO在kvm和qemu之间执行过程,同时描述kvm和qemu之间如何协同工作的.
参考资料:
1. qemu-kvm的初始化与客户系统的执行:http://blog.csdn.net/lux_veritas/article/details/9383643
2. 内核虚拟化kvm/qemu----guest os,kvm,qemu工作流程:http://www.360doc.com/content/12/0619/13/7982302_219186951.shtml
原文地址：http://blog.csdn.net/dashulu/article/details/17074675