virtio的原理说起来挺简单的，两边是front-driver，backend-driver，中间是virtio和vring。关键是front-driver如何将io操作传到backend-driver，backend-driver又是如何将这些io操作解析出来，执行完，通知回front-driver的？这里面的细节，细思极恐！
问题一：Guest，Hypervisor，Host是如何建立联系的？本质就是初始化过程！
virtio初始化函数很有规律基本就是：virtio_*_init，这种形式，那就好搞了，用systemtap抓取初始化的调用：
各种class初始化：
call virtio_device_class_init
call virtio_blk_class_init
call virtio_rng_class_init
call virtio_serial_port_class_init
call virtio_pci_class_init
call virtio_serial_pci_class_init
call virtio_bus_class_init
call virtio_pci_bus_class_init
call virtio_scsi_pci_class_init
call virtio_balloon_pci_class_init
call virtio_serial_class_init
call virtio_balloon_class_init
call virtio_mmio_bus_class_init
call virtio_scsi_common_class_init
call virtio_net_class_init
call virtio_mmio_class_init
call virtio_9p_class_init
call virtio_blk_pci_class_init
call virtio_scsi_class_init
call virtio_9p_pci_class_init
call virtio_rng_pci_class_init
call virtio_net_pci_class_init
virtio-serial-pci初始化（这个是个什么设备）：
call virtio_serial_pci_instance_init
call virtio_pci_init
call virtio_serial_pci_init
我们的主角virtio blk初始化：
call virtio_blk_pci_instance_init
call virtio_pci_init
call virtio_blk_pci_init
我在虚拟机里面添加了virtio balloon设备：
call virtio_balloon_pci_instance_init
call virtio_pci_init
call virtio_balloon_pci_init
virtio blk的Qemu相关参数有两个：
-drive file=/home/mq/Documents/IDV/idv-update/login.img,if=none,id=drive-virtio-disk1,format=qcow2
-device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x8,drive=drive-virtio-disk1,id=virtio-disk1,bootindex=1
可以看到virtio blk的TypeInfo name是："virtio-blk-pci"。
Instance结构：
struct PCIDevice {
DeviceState qdev;
/* PCI config space */
uint8_t *config;
/* Used to enable config checks on load. Note that writable bits are
* never checked even if set in cmask. */
uint8_t *cmask;
/* Used to implement R/W bytes */
uint8_t *wmask;
/* Used to implement RW1C(Write 1 to Clear) bytes */
uint8_t *w1cmask;
/* Used to allocate config space for capabilities. */
uint8_t *used;
/* the following fields are read only */
PCIBus *bus;
int32_t devfn;
/* Cached device to fetch requester ID from, to avoid the PCI
* tree walking every time we invoke PCI request (e.g.,
* MSI). For conventional PCI root complex, this field is
* meaningless. */
PCIReqIDCache requester_id_cache;
char name[64];
PCIIORegion io_regions[PCI_NUM_REGIONS];
AddressSpace bus_master_as;
MemoryRegion bus_master_container_region;
MemoryRegion bus_master_enable_region;
/* do not access the following fields */
PCIConfigReadFunc *config_read;
PCIConfigWriteFunc *config_write;
/* Legacy PCI VGA regions */
MemoryRegion *vga_regions[QEMU_PCI_VGA_NUM_REGIONS];
bool has_vga;
/* Current IRQ levels. Used internally by the generic PCI code. */
uint8_t irq_state;
/* Capability bits */
uint32_t cap_present;
/* Offset of MSI-X capability in config space */
uint8_t msix_cap;
/* MSI-X entries */
int msix_entries_nr;
/* Space to store MSIX table & pending bit array */
uint8_t *msix_table;
uint8_t *msix_pba;
/* MemoryRegion container for msix exclusive BAR setup */
MemoryRegion msix_exclusive_bar;
/* Memory Regions for MSIX table and pending bit entries. */
MemoryRegion msix_table_mmio;
MemoryRegion msix_pba_mmio;
/* Reference-count for entries actually in use by driver. */
unsigned *msix_entry_used;
/* MSIX function mask set or MSIX disabled */
bool msix_function_masked;
/* Version id needed for VMState */
int32_t version_id;
/* Offset of MSI capability in config space */
uint8_t msi_cap;
/* PCI Express */
PCIExpressDevice exp;
/* SHPC */
SHPCDevice *shpc;
/* Location of option rom */
char *romfile;
bool has_rom;
MemoryRegion rom;
uint32_t rom_bar;
/* INTx routing notifier */
PCIINTxRoutingNotifier intx_routing_notifier;
/* MSI-X notifiers */
MSIVectorUseNotifier msix_vector_use_notifier;
MSIVectorReleaseNotifier msix_vector_release_notifier;
MSIVectorPollNotifier msix_vector_poll_notifier;
};
struct VirtIOPCIProxy {
PCIDevice pci_dev;
MemoryRegion bar;
VirtIOPCIRegion common;
VirtIOPCIRegion isr;
VirtIOPCIRegion device;
VirtIOPCIRegion notify;
VirtIOPCIRegion notify_pio;
MemoryRegion modern_bar;
MemoryRegion io_bar;
MemoryRegion modern_cfg;
AddressSpace modern_as;
uint32_t legacy_io_bar_idx;
uint32_t msix_bar_idx;
uint32_t modern_io_bar_idx;
uint32_t modern_mem_bar_idx;
int config_cap;
uint32_t flags;
bool disable_modern;
bool ignore_backend_features;
OnOffAuto disable_legacy;
uint32_t class_code;
uint32_t nvectors;
uint32_t dfselect;
uint32_t gfselect;
uint32_t guest_features[2];
VirtIOPCIQueue vqs[VIRTIO_QUEUE_MAX];
VirtIOIRQFD *vector_irqfd;
int nvqs_with_notifiers;
VirtioBusState bus;
};
struct VirtIOBlkPCI {
VirtIOPCIProxy parent_obj;
VirtIOBlock vdev;
};
上面的结构体有个规律，逐渐由pci device往blk device转变，VirtIOPCIProxy，VirtIOBlock两个就是pci device和blk device的桥梁。
