qemu/kvm学习笔记

qemu/kvm架构

cpu虚拟化的示例

Reference: kvmtest.c [LWN.net]

主要步骤：

1. QEMU通过/dev/kvm设备文件发起KVM_CREATE_VM ioctl，请求KVM创建一个虚拟机。KVM创建虚拟机相应的结构体，并为QEMU返回一个虚拟机文件描述符
2. QEMU通过虚拟机文件描述符发起KVM_CREATE_VCPU ioctl，请求KVM创建一个vCPU。KVM创建vCPU相应的结构体并初始化，返回一个vCPU文件描述符。
3. QEMU通过vCPU文件描述符发起KVM_RUN ioctl，vCPU线程执行VMLAUNCH指令进入非根模式，执行虚拟机代码直至发生VM-Exit。
4. KVM根据VM-Exit的原因进行相应处理，如果与IO有关，则需要进一步返回到QEMU中进行处理。

运行结果： 

代码实现：

/* Sample code for /dev/kvm API */

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

int main(void)
{
    int kvm, vmfd, vcpufd, ret;
    const uint8_t code[] = {
        /* 写入指定端口 0x3f8，输出 Hello */
        0xba, 0xf8, 0x03, /* mov $0x3f8, %dx */
        0x00, 0xd8,       /* add %bl, %al */
        0x04, '0',        /* add $'0', %al */
        0xee,             /* out %al, (%dx) */
        0xb0, '\n',       /* mov $'\n', %al */
        0xee,             /* out %al, (%dx) */

        0xb0, 'H',        /* mov $'H', %al */
        0xee,             /* out %al, (%dx) */
        0xb0, 'e',        /* mov $'e', %al */
        0xee,             /* out %al, (%dx) */
        0xb0, 'l',        /* mov $'l', %al */
        0xee,             /* out %al, (%dx) */
        0xb0, 'l',        /* mov $'l', %al */
        0xee,             /* out %al, (%dx) */
        0xb0, 'o',        /* mov $'o', %al */
        0xee,             /* out %al, (%dx) */
        0xb0, '\n',       /* mov $'\n', %al */
        0xee,             /* out %al, (%dx) */
        0xf4,             /* hlt */
    };
    uint8_t *mem;
    struct kvm_sregs sregs;
    size_t mmap_size;
    struct kvm_run *run;

    // ** step 1. 打开 KVM 模块设备文件
    kvm = open("/dev/kvm", O_RDWR | O_CLOEXEC);
    if (kvm == -1)
        err(1, "/dev/kvm");

    // 获取 KVM API 版本
    /* Make sure we have the stable version of the API */
    ret = ioctl(kvm, KVM_GET_API_VERSION, NULL);
    if (ret == -1)
        err(1, "KVM_GET_API_VERSION");
    if (ret != 12)
        errx(1, "KVM_GET_API_VERSION %d, expected 12", ret);

    // ** step 2. KVM_CREATE_VM 创建虚拟机获得虚拟机文件描述符
    vmfd = ioctl(kvm, KVM_CREATE_VM, (unsigned long)0);
    if (vmfd == -1)
        err(1, "KVM_CREATE_VM");

    // 分配 4KB 内存空间存放二进制代码
    // 这里的 0x1000(HVA)
    /* Allocate one aligned page of guest memory to hold the code. */
    mem = mmap(NULL, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
    if (!mem)
        err(1, "allocating guest memory");
    // 将二进制代码复制至分配的内存页中
    memcpy(mem, code, sizeof(code));

    // KVM_SET_USER_MEMORY_REGION 将该内存页映射至虚拟机物理地址 0x1000(GPA) 处
    /* Map it to the second page frame (to avoid the real-mode IDT at 0). */
    struct kvm_userspace_memory_region region = {
        .slot = 0,
        .guest_phys_addr = 0x1000,
        .memory_size = 0x1000,
        .userspace_addr = (uint64_t)mem,
    };
    ret = ioctl(vmfd, KVM_SET_USER_MEMORY_REGION, ®ion);
    if (ret == -1)
        err(1, "KVM_SET_USER_MEMORY_REGION");

    // ** step 3. KVM_CREATE_VCPU 创建 vCPU 获得 vCPU 文件描述符
    vcpufd = ioctl(vmfd, KVM_CREATE_VCPU, (unsigned long)0);
    if (vcpufd == -1)
        err(1, "KVM_CREATE_VCPU");

    // ** step 4. 获取 QEMU/KVM 共享内存空间大小，并映射 kvm_run 结构体
    /* Map the shared kvm_run structure and following data. */
    ret = ioctl(kvm, KVM_GET_VCPU_MMAP_SIZE, NULL);
    if (ret == -1)
        err(1, "KVM_GET_VCPU_MMAP_SIZE");
    mmap_size = ret;
    if (mmap_size < sizeof(*run))
        errx(1, "KVM_GET_VCPU_MMAP_SIZE unexpectedly small");
    // 使用 vCPU 文件描述符
    // 映射 kvm_run 结构体
    run = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, vcpufd, 0);
    if (!run)
        err(1, "mmap vcpu");

    // ** step 5. 设置 CS 寄存器和 RIP 寄存器，使得 vCPU 从 0x1000 处开始执行
    /* Initialize CS to point at 0, via a read-modify-write of sregs. */
    ret = ioctl(vcpufd, KVM_GET_SREGS, &sregs);
    if (ret == -1)
        err(1, "KVM_GET_SREGS");
    sregs.cs.base = 0;
    sregs.cs.selector = 0;
    ret = ioctl(vcpufd, KVM_SET_SREGS, &sregs);
    if (ret == -1)
        err(1, "KVM_SET_SREGS");

    /* Initialize registers: instruction pointer for our code, addends, and
     * initial flags required by x86 architecture. */
    struct kvm_regs regs = {
        .rip = 0x1000,
        .rax = 2,
        .rbx = 2,
        .rflags = 0x2,
    };
    ret = ioctl(vcpufd, KVM_SET_REGS, ®s);
    if (ret == -1)
        err(1, "KVM_SET_REGS");

    /* Repeatedly run code and handle VM exits. */
    while (1) {
        // ** step 6. KVM_RUN 运行 vCPU
        ret = ioctl(vcpufd, KVM_RUN, NULL);
        if (ret == -1)
            err(1, "KVM_RUN");

        // ** step 7. 处理 VM-Exit
        switch (run->exit_reason) {
        case KVM_EXIT_HLT: // hlt 指令触发 VM-Exit
            puts("KVM_EXIT_HLT");
            return 0; // 退出程序

        case KVM_EXIT_IO: // 依次调用 out 指令向 0x3f8 端口写入字符时，会触发 VM-Exit，使得程序返回到用户态处理
            // 输出写入 0x3f8 端口的字符
            if (run->io.direction == KVM_EXIT_IO_OUT && run->io.size == 1 && run->io.port == 0x3f8 && run->io.count == 1)
                // 调用 putchar 函数输出字符
                putchar(*(((char *)run) + run->io.data_offset));
            else
                errx(1, "unhandled KVM_EXIT_IO");
            break;
        case KVM_EXIT_FAIL_ENTRY:
            errx(1, "KVM_EXIT_FAIL_ENTRY: hardware_entry_failure_reason = 0x%llx",
                 (unsigned long long)run->fail_entry.hardware_entry_failure_reason);
        case KVM_EXIT_INTERNAL_ERROR:
            errx(1, "KVM_EXIT_INTERNAL_ERROR: suberror = 0x%x", run->internal.suberror);
        default:
            errx(1, "exit_reason = 0x%x", run->exit_reason);
        }
    }
}

KVM API

/usr/include/linux/kvm.h 

ioctl	KVM API	Description	Example
ioctls for /dev/kvm fds	KVM_GET_API_VERSION	获取 KVM API 版本	kvm = open("/dev/kvm", O_RDWR | O_CLOEXEC) ret = ioctl(kvm, KVM_GET_API_VERSION, NULL)
	KVM_CREATE_VM	创建虚拟机获得虚拟机文件描述符	vmfd = ioctl(kvm, KVM_CREATE_VM, 0)
ioctls for VM fds	KVM_SET_USER_MEMORY_REGION	将内存页映射至虚拟机物理地址处	ret = ioctl(vmfd, KVM_SET_USER_MEMORY_REGION, ®ion)
	KVM_CREATE_VCPU	创建 vCPU 获得 vCPU 文件描述符	vcpufd = ioctl(vmfd, KVM_CREATE_VCPU, 0)
	KVM_GET_VCPU_MMAP_SIZE	获取 QEMU/KVM 共享内存空间大小	mmap_size = ioctl(kvm, KVM_GET_VCPU_MMAP_SIZE, NULL); 使用 vCPU 文件描述符，映射 kvm_run 结构体 run = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_SHARED, vcpufd, 0)
ioctls for vcpu fds	KVM_GET_SREGS	获取 CS 寄存器和 RIP 寄存器	ret = ioctl(vcpufd, KVM_GET_SREGS, &sregs)
	KVM_SET_SREGS	设置 CS 寄存器和 RIP 寄存器	sregs.cs.base = 0 sregs.cs.selector = 0 ret = ioctl(vcpufd, KVM_SET_SREGS, &sregs) struct kvm_regs regs = { .rip = 0x1000, .rax = 2, .rbx = 2, .rflags = 0x2, } ret = ioctl(vcpufd, KVM_SET_REGS, ®s)
	KVM_RUN	运行 vCPU	ret = ioctl(vcpufd, KVM_RUN, NULL)