vhost是virtio的另一种方案，用于跳过qemu，减少qemu和内核之间上下文切换的开销，对于网络IO而言提升尤其明显。vhost目前有两种实现方案，内核态和用户态，本文重点讨论内核态的vhost

vhost内核模块主要处理数据面的事情，控制面上还是交给qemu，vhost的数据结构如下

struct vhost_dev {
    MemoryListener memory_listener;  /* MemoryListener是物理内存操作的回调函数集合 */
    struct vhost_memory *mem;
    int n_mem_sections;
    MemoryRegionSection *mem_sections;
    struct vhost_virtqueue *vqs;  /* vhost_virtqueue列表和个数 */
    int nvqs;
    /* the first virtuque which would be used by this vhost dev */
    int vq_index;
    unsigned long long features;  /* vhost设备支持的features */
    unsigned long long acked_features;  /* guest acked的features */
    unsigned long long backend_features;  /* backend, e.g. tap设备，支持的features */
    bool started;
    bool log_enabled;
    vhost_log_chunk_t *log;
    unsigned long long log_size;
    Error *migration_blocker;
    bool force;
    bool memory_changed;
    hwaddr mem_changed_start_addr;
    hwaddr mem_changed_end_addr;
    const VhostOps *vhost_ops; /* VhostOps基于kernel和user两种形态的vhost有不同的实现，内核的实现最终调用ioctl完成 */
    void *opaque;
};
 
struct vhost_virtqueue {
    int kick;
    int call;
    void *desc;
    void *avail;
    void *used;
    int num;
    unsigned long long used_phys;
    unsigned used_size;
    void *ring;
    unsigned long long ring_phys;
    unsigned ring_size;
    EventNotifier masked_notifier;
};
vhost的内存布局，也是由一组vhost_memory_region构成，
struct vhost_memory_region {
    __u64 guest_phys_addr;
    __u64 memory_size; /* bytes */
    __u64 userspace_addr;
    __u64 flags_padding; /* No flags are currently specified. */
};
 
/* All region addresses and sizes must be 4K aligned. */
#define VHOST_PAGE_SIZE 0x1000
 
struct vhost_memory {
    __u32 nregions;
    __u32 padding;
    struct vhost_memory_region regions[0];
};

static const VhostOps kernel_ops = {
    .backend_type = VHOST_BACKEND_TYPE_KERNEL,
    ....
    .vhost_set_mem_table = vhost_kernel_set_mem_table,
    .vhost_set_vring_addr = vhost_kernel_set_vring_addr,
    ....
}
 
const VhostOps user_ops = {
    .backend_type = VHOST_BACKEND_TYPE_USER,
    ...
    .vhost_set_mem_table = vhost_user_set_mem_table,
    .vhost_set_vring_addr = vhost_user_set_vring_addr,
    ...