在上一篇文章中，我们阐述了KVM中对/dev/kvm的调用逻辑，当通过系统调用ioctl(/dev/kvm, KVM_CREATE_VM)时，会通过带哦用kvm_dev_ioctl函数进而调用kvm_dev_ioctl_create_vm
static int kvm_dev_ioctl_create_vm(unsigned long type)
{
int r;
struct kvm *kvm;
struct file *file;
kvm = kvm_create_vm(type);
if (IS_ERR(kvm))
return PTR_ERR(kvm);
#ifdef CONFIG_KVM_MMIO
r = kvm_coalesced_mmio_init(kvm);
if (r < 0)
goto put_kvm;
#endif
r = get_unused_fd_flags(O_CLOEXEC);
if (r < 0)
goto put_kvm;
file = anon_inode_getfile("kvm-vm", &kvm_vm_fops, kvm, O_RDWR);
if (IS_ERR(file)) {
put_unused_fd(r);
r = PTR_ERR(file);
goto put_kvm;
}
/*
* Don't call kvm_put_kvm anymore at this point; file->f_op is
* already set, with ->release() being kvm_vm_release(). In error
* cases it will be called by the final fput(file) and will take
* care of doing kvm_put_kvm(kvm).
*/
if (kvm_create_vm_debugfs(kvm, r) < 0) {
put_unused_fd(r);
fput(file);
return -ENOMEM;
}
kvm_uevent_notify_change(KVM_EVENT_CREATE_VM, kvm);
fd_install(r, file);
return r;
put_kvm:
kvm_put_kvm(kvm);
return r;
}
kvm_dev_ioctl_create_vm函数的主要作用就是创建虚拟机实例，每个虚拟机实例用一个kvm结构体表示，kvm_coalesced_mmio_init主要用来初始化MMIO，anon_inode_getfile创建一个匿名file,其中file_operations就是kvm_vm_fops，私有数据就是刚刚创建的虚拟机，这个file对应的fs返回给用户态的QEMU，表示一台虚拟机，QEMU之后就可以通过该fd对虚拟机进行操作了。
static struct file_operations kvm_vm_fops = {
.release = kvm_vm_release,
.unlocked_ioctl = kvm_vm_ioctl,
.llseek = noop_llseek,
KVM_COMPAT(kvm_vm_compat_ioctl),
};
接下来重点分析kvm_create_vm函数，该函数是创建虚拟机的核心函数，部分代码如下：
static struct kvm *kvm_create_vm(unsigned long type)
{
struct kvm *kvm = kvm_arch_alloc_vm();
int r = -ENOMEM;
int i;
if (!kvm)
return ERR_PTR(-ENOMEM);
spin_lock_init(&kvm->mmu_lock);
mmgrab(current->mm);
kvm->mm = current->mm;
kvm_eventfd_init(kvm);
mutex_init(&kvm->lock);
mutex_init(&kvm->irq_lock);
mutex_init(&kvm->slots_lock);
INIT_LIST_HEAD(&kvm->devices);
BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
struct kvm_memslots *slots = kvm_alloc_memslots();
if (!slots)
goto out_err_no_arch_destroy_vm;
/* Generations must be different for each address space. */
slots->generation = i;
rcu_assign_pointer(kvm->memslots[i], slots);
}
r = kvm_arch_init_vm(kvm, type);
if (r)
goto out_err_no_arch_destroy_vm;
r = hardware_enable_all();
if (r)
goto out_err_no_disable;
r = kvm_init_mmu_notifier(kvm);
if (r)
goto out_err_no_mmu_notifier;
r = kvm_arch_post_init_vm(kvm);
if (r)
goto out_err;
mutex_lock(&kvm_lock);
list_add(&kvm->vm_list, &vm_list);
mutex_unlock(&kvm_lock);
preempt_notifier_inc();
return kvm;
kvm_arch_alloc_vm分配一个KVM结构体，用来表示一台虚拟机，该函数最终根据CPU类型调用vmx/svm的vmx_vm_alloc或svm_vm_alloc函数，本文以intelCPU为例，因此调用vmx_vm_alloc函数分配内存。
static inline struct kvm *kvm_arch_alloc_vm(void)
{
return kvm_x86_ops->vm_alloc();
}
static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
.cpu_has_kvm_support = cpu_has_kvm_support,
.disabled_by_bios = vmx_disabled_by_bios,
.hardware_setup = hardware_setup,
.hardware_unsetup = hardware_unsetup,
.check_processor_compatibility = vmx_check_processor_compat,
.hardware_enable = hardware_enable,
.hardware_disable = hardware_disable,
.cpu_has_accelerated_tpr = report_flexpriority,
.has_emulated_msr = vmx_has_emulated_msr,
.vm_init = vmx_vm_init,
.vm_alloc = vmx_vm_alloc,
}
static struct kvm *vmx_vm_alloc(void)
{
struct kvm_vmx *kvm_vmx = __vmalloc(sizeof(struct kvm_vmx),
GFP_KERNEL_ACCOUNT | __GFP_ZERO,
PAGE_KERNEL);
return &kvm_vmx->kvm;
}
kvm->mm最终指向current->mm，即kvm的mm结构指向qemu进程的虚拟内存，也就是虚拟机的物理内存就是qemu进程的虚拟内存。我们用真实的vm实例看下qemu进程的虚拟内存大小是不是虚拟机的物理内存大小.
kvm结构体定义在文件include/linux/kvm_host.h
struct kvm {
spinlock_t mmu_lock;
struct mutex slots_lock;
struct mm_struct *mm; /* userspace tied to this vm */
struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
/*
* created_vcpus is protected by kvm->lock, and is incremented
* at the beginning of KVM_CREATE_VCPU. online_vcpus is only
* incremented after storing the kvm_vcpu pointer in vcpus,
* and is accessed atomically.
*/
atomic_t online_vcpus;
int created_vcpus;
int last_boosted_vcpu;
struct list_head vm_list;
struct mutex lock;
struct kvm_io_bus __rcu *buses[KVM_NR_BUSES];
#ifdef CONFIG_HAVE_KVM_EVENTFD
struct {
spinlock_t lock;
struct list_head items;
struct list_head resampler_list;
struct mutex resampler_lock;
} irqfds;
struct list_head ioeventfds;
#endif
struct kvm_vm_stat stat;
struct kvm_arch arch;
refcount_t users_count;
#ifdef CONFIG_KVM_MMIO
struct kvm_coalesced_mmio_ring *coalesced_mmio_ring;
spinlock_t ring_lock;
struct list_head coalesced_zones;
#endif
struct mutex irq_lock;
#ifdef CONFIG_HAVE_KVM_IRQCHIP
/*
* Update side is protected by irq_lock.
*/
struct kvm_irq_routing_table __rcu *irq_routing;
#endif
#ifdef CONFIG_HAVE_KVM_IRQFD
struct hlist_head irq_ack_notifier_list;
#endif
#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
struct mmu_notifier mmu_notifier;
unsigned long mmu_notifier_seq;
long mmu_notifier_count;
#endif
long tlbs_dirty;
struct list_head devices;
bool manual_dirty_log_protect;
struct dentry *debugfs_dentry;
struct kvm_stat_data **debugfs_stat_data;
struct srcu_struct srcu;
struct srcu_struct irq_srcu;
pid_t userspace_pid;
};
KVM有个类型为kvm_arch的arch成员，用于存放与架构相关的数据，kvm_arch_init_vm用来初始化这些数据。紧接着调用hardware_enable_all函数来开启vmx模式，hardware_enable_all在创建第一个虚拟机的时候对每个CPU调用hardware_enable_nolock
static int hardware_enable_all(void)
{
int r = 0;
raw_spin_lock(&kvm_count_lock);
kvm_usage_count++;
if (kvm_usage_count == 1) {
atomic_set(&hardware_enable_failed, 0);
on_each_cpu(hardware_enable_nolock, NULL, 1);
if (atomic_read(&hardware_enable_failed)) {
hardware_disable_all_nolock();
r = -EBUSY;
}
}
raw_spin_unlock(&kvm_count_lock);
return r;
}
static void hardware_enable_nolock(void *junk)
{
int cpu = raw_smp_processor_id();
int r;
if (cpumask_test_cpu(cpu, cpus_hardware_enabled))
return;
cpumask_set_cpu(cpu, cpus_hardware_enabled);
r = kvm_arch_hardware_enable();
if (r) {
cpumask_clear_cpu(cpu, cpus_hardware_enabled);
atomic_inc(&hardware_enable_failed);
pr_info("kvm: enabling virtualization on CPU%d failed\n", cpu);
}
}
int kvm_arch_hardware_enable(void)
{
struct kvm *kvm;
struct kvm_vcpu *vcpu;
int i;
int ret;
u64 local_tsc;
u64 max_tsc = 0;
bool stable, backwards_tsc = false;
kvm_shared_msr_cpu_online();
ret = kvm_x86_ops->hardware_enable();
if (ret != 0)
return ret;
local_tsc = rdtsc();
stable = !kvm_check_tsc_unstable();
list_for_each_entry(kvm, &vm_list, vm_list) {
kvm_for_each_vcpu(i, vcpu, kvm) {
if (!stable && vcpu->cpu == smp_processor_id())
kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
if (stable && vcpu->arch.last_host_tsc > local_tsc) {
backwards_tsc = true;
if (vcpu->arch.last_host_tsc > max_tsc)
max_tsc = vcpu->arch.last_host_tsc;
}
}
}
if (backwards_tsc) {
u64 delta_cyc = max_tsc - local_tsc;
list_for_each_entry(kvm, &vm_list, vm_list) {
kvm->arch.backwards_tsc_observed = true;
kvm_for_each_vcpu(i, vcpu, kvm) {
vcpu->arch.tsc_offset_adjustment += delta_cyc;
vcpu->arch.last_host_tsc = local_tsc;
kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
}
/*
* We have to disable TSC offset matching.. if you were
* booting a VM while issuing an S4 host suspend....
* you may have some problem. Solving this issue is
* left as an exercise to the reader.
*/
kvm->arch.last_tsc_nsec = 0;
kvm->arch.last_tsc_write = 0;
}
}
return 0;
}
同样在kvm_x86_ops->hardware_enable();会调用vmx的hardware_enable函数，该函数主要作用就是设置CR4的VMXE位并且调用VMXON指令开启VMX。
static int hardware_enable(void)
{
int cpu = raw_smp_processor_id();
u64 phys_addr = __pa(per_cpu(vmxarea, cpu));
u64 old, test_bits;
if (cr4_read_shadow() & X86_CR4_VMXE)
return -EBUSY;
/*
* This can happen if we hot-added a CPU but failed to allocate
* VP assist page for it.
*/
if (static_branch_unlikely(&enable_evmcs) &&
!hv_get_vp_assist_page(cpu))
return -EFAULT;
INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
crash_enable_local_vmclear(cpu);
rdmsrl(MSR_IA32_FEATURE_CONTROL, old);
test_bits = FEATURE_CONTROL_LOCKED;
test_bits |= FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX;
if (tboot_enabled())
test_bits |= FEATURE_CONTROL_VMXON_ENABLED_INSIDE_SMX;
if ((old & test_bits) != test_bits) {
/* enable and lock */
wrmsrl(MSR_IA32_FEATURE_CONTROL, old | test_bits);
}
kvm_cpu_vmxon(phys_addr);
if (enable_ept)
ept_sync_global();
return 0;
}
还有kvm_alloc_memslots以及kvm_init_mmu_notifier函数，这些与内存相关的会放在以后阐述，主要是和内存虚拟化相关。preempt_notifier_inc主要将VCPU线程进行调度。
以上就是对kvm_create_vm函数的阐述，当然里面还有很多细节没有深挖，虚拟化细节相关的我也在逐步了解，有些可能需要等了解到之后再来阐述了。
每天进步一小步，未来进步一大步！
