virtio 是一种 I/O 半虚拟化解决方案，是一套通用 I/O 设备虚拟化的程序，是对半虚拟化 Hypervisor 中的一组通用 I/O 设备的抽象。对比其他设备有宿主计算机模拟，virtio设备效率更高。

virtio架构图：

最上面一排是不同的设备，如块设备，网络设备，控制台等

virtio 层属于控制层，负责设备跟宿主OS之间的通知机制（kick，notify）和控制流程，而 virtio-vring 则负责具体数据流转发。

vring 包含三个部分，描述符数组 desc，可用的 available ring 和使用过的 used ring。

struct vring_desc {/* Address (guest-physical). */u64 addr;/* Length. */u32 len;/* The flags as indicated above. */u16 flags;/* We chain unused descriptors via this, too */u16 next;
};

vring描述符结构。

struct vring_avail {u16 flags;u16 idx;u16 ring[];
};
struct vring_used_elem {/* Index of start of used descriptor chain. */u32 id;/* Total length of the descriptor chain which was used (written to) */u32 len;
};struct vring_used {u16 flags;u16 idx;struct vring_used_elem ring[];
};

avail跟used结构。

static inline void vring_init(struct vring *vr, unsigned int num, void *p,unsigned long align)
{vr->num = num;vr->desc = p;vr->avail = (void *)((char *)p + num*sizeof(struct vring_desc));vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + sizeof(u16)+ align-1) & ~(align - 1));
}static inline unsigned vring_size(unsigned int num, unsigned long align)
{return ((sizeof(struct vring_desc) * num + sizeof(u16) * (3 + num)+ align - 1) & ~(align - 1))+ sizeof(u16) * 3 + sizeof(struct vring_used_elem) * num;
}

vring_size计算vring需要的内存大小，num是vring的描述符数量，为2^N，如128，256，512等。

首先放的是num个描述符vring_desc，然后是vring_avail，在vring_avail结尾有个16字位的used_event，因此是是（3+num）。之后放得是vring_used，同样，后面有个16位的avail_event，因此也是sizeof(u16) * 3。

vring_used需要4K对齐。

used_event用来通知宿主os，读取到哪里了。avail_event用来通知宿主os，写到哪里了。

设置avail_event，用来通知宿主os有新命令需要处理。宿主os完成之后产生中断，处理完成之后设置used_event，表示数据处理过了，这样宿主os才会有数据的时候继续发生中断。

desc 用于存储一些关联的描述符，每个描述符记录一个对 buffer 的描述，available ring 则用于 guest 端表示当前有哪些描述符是可用的，而 used ring 则表示 host 端哪些描述符已经被使用。

Virtio 使用 virtqueue 来实现 I/O 机制，每个 virtqueue 就是一个承载大量数据的队列，具体使用多少个队列取决于需求，例如，virtio 网络驱动程序（virtio-net）使用两个队列（一个用于接受，另一个用于发送），而 virtio 块驱动程序（virtio-blk）仅使用一个队列。

比如读取硬盘。取得virtqueue队列，virtio-blk就一个队列。然后往里面添加3条结构化数据。

struct addr_size {unsigned long  vp_addr;           /* 物理地址 */u32 vp_size;               /* 大小 */u32 vp_flag;   /*标记，如读，写*/
};

第一条是请求命令数据：

 struct blk_outhdr {/* VIRTIO_BLK_T* */u32 type;/* io priority. */u32 ioprio;/* Sector (ie. 512 byte offset) */u64 sector;};

告诉驱动是读还是写（type），优先级，扇区号。

第二条是输出缓冲区，即扇区读取到哪里，缓冲区大小。

第三条就1个字节，用来指示操作结果，0表示成功。

module/blk/virtio_blk.c

struct blk_req {struct blk_outhdr hdr;uchar status;
};
static struct blk_req rq;
static void virtio_blk_read(ulong sector)
{struct addr_size phys[3];rq.hdr.type = VIRTIO_BLK_T_IN;rq.hdr.ioprio = 0;rq.hdr.sector = sector;phys[0].vp_addr = V2P((ulong)&rq.hdr);phys[0].vp_size = sizeof(rq.hdr);phys[0].vp_flag = VRING_DESC_F_READ;phys[1].vp_addr = V2P((ulong)buf);phys[1].vp_size = 512;phys[1].vp_flag = VRING_DESC_F_WRITE;uchar status = 0;phys[2].vp_addr = V2P((ulong)&rq.status);phys[2].vp_size = sizeof(status);pyhs[2].vp_flag = VRING_DESC_F_WRITE;virtio_to_queue(to_virtio_dev_t(&pci_vblk), 0, phys, 3, &rq);}

virtio_to_queue把3个描述符写入vring，然后kick通知宿主os。

libs/libvirtio/virtio.c

int
virtio_to_queue(virtio_dev_t dev, int qidx, struct addr_size *bufs,size_t num, void *data)
{u16_t free_first;int left;struct virtio_queue *q = &dev->queues[qidx];struct vring *vring = &q->vring;ASSERT(0 <= qidx && qidx <= dev->num_queues);if (!data)panic("%s: NULL data received queue %d", __func__, qidx);free_first = q->free_head;left = (int)q->free_num - (int)num;pci_d("free_first:%016lx,left:%d,dev->threads:%d,n:%d\n",free_first,left, dev->threads,num);if (left < dev->threads)set_indirect_descriptors(dev, q, bufs, num);elseset_direct_descriptors(q, bufs, num);/* Next index for host is old free_head */vring->avail->ring[vring->avail->idx % q->num] = free_first;/* Provided by the caller to identify this slot */q->data[free_first] = data;/* Make sure the host sees the new descriptors */virtio_wmb(true);/* advance last idx */vring->avail->idx += 1;/* Make sure the host sees the avail->idx */virtio_rmb(true);/* kick it! */kick_queue(dev, qidx);return 0;
}

kick_queue通知宿主os进行处理。

void *data为驱动数据，到时候宿主os完成之后通知驱动调用virtio_from_queue会返回。

这里为struct blk_req rq; 目前比较简单，就一个状态跟hdr请求结构。状态为第三个描述符：

    phys[2].vp_addr = V2P((ulong)&rq.status);phys[2].vp_size = sizeof(status);pyhs[2].vp_flag = VRING_DESC_F_WRITE;

由宿主os填写是否操作成功。

int virtio_from_queue(virtio_dev_t dev, int qidx, void **data, size_t * len)
{struct virtio_queue *q;struct vring *vring;struct vring_used_elem *uel;struct vring_desc *vd;int count = 0;u16_t idx;u16_t used_idx;ASSERT(0 <= qidx && qidx < dev->num_queues);q = &dev->queues[qidx];vring = &q->vring;/* Make sure we see changes done by the host */virtio_rmb(true);/* The index from the host */used_idx = vring->used->idx % q->num;/* We already saw this one, nothing to do here */if (q->last_used == used_idx)return -1;/* Get the vring_used element */uel = &q->vring.used->ring[q->last_used];/* Update the last used element */q->last_used = (q->last_used + 1) % q->num;/* index of the used element */idx = uel->id % q->num;ASSERT(q->data[idx] != NULL);/* Get the descriptor */vd = &vring->desc[idx];/* Unconditionally set the tail->next to the first used one */ASSERT(vring->desc[q->free_tail].flags & VRING_DESC_F_NEXT);vring->desc[q->free_tail].next = idx;/* Find the last index, eventually there has to be one* without a the next flag.** FIXME: Protect from endless loop*/while (vd->flags & VRING_DESC_F_NEXT) {if (vd->flags & VRING_DESC_F_INDIRECT)clear_indirect_table(dev, vd);idx = vd->next;vd = &vring->desc[idx];count++;}/* Didn't count the last one */count++;if (vd->flags & VRING_DESC_F_INDIRECT)clear_indirect_table(dev, vd);/* idx points to the tail now, update the queue */q->free_tail = idx;ASSERT(!(vd->flags & VRING_DESC_F_NEXT));/* We can always connect the tail with the head */vring->desc[q->free_tail].next = q->free_head;vring->desc[q->free_tail].flags = VRING_DESC_F_NEXT;q->free_num += count;ASSERT(q->free_num <= q->num);*data = q->data[uel->id];q->data[uel->id] = NULL;if (len != NULL)*len = uel->len;q->last_used_idx++;if (!(vring->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))virtio_store_mb(true,vring->used_event,q->last_used_idx);return 0;
}

virtio_from_queue用于处理结果。

下面做个virtio_blk的实验:

module/blk/virtio_blk.c

static void irq_handler(int n)
{uchar isr =virtio_conf_readb(to_virtio_dev_t(&pci_vblk), VIRTIO_PCI_ISR);ack_lapic_irq();virtio_queue_disable_intr(virtio_get_queue(to_virtio_dev_t(&pci_vblk), 0));printk("****virtio_blk_irq_handler**** irq:%d,isr:%d cpu:%d\n", n,isr,smp_processor_id());virtio_blk_check_queue();
}
void virtio_blk_read_config(struct virtio_blk *p)
{virtio_dev_t pvirtio = to_virtio_dev_t(p);u32 offset = virtio_pci_config_offset(to_virtio_dev_t(p));virtio_conf_read(pvirtio, offset, &p->_config,sizeof(p->_config));if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_SIZE_MAX)) {pci_d("VIRTIO_BLK_F_SIZE_MAX:%x\n",p->_config.size_max);}if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_SEG_MAX)) {pci_d("VIRTIO_BLK_F_SEG_MAX:%x\n",p->_config.seg_max);}if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_GEOMETRY)) {pci_d("VIRTIO_BLK cylinders:%x,heads:%x,sectors:%x\n",p->_config.geometry.cylinders,p->_config.geometry.heads,p->_config.geometry.sectors);}if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_BLK_SIZE)) {pci_d("VIRTIO_BLK_F_BLK_SIZE:%x\n",p->_config.blk_size);}if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_TOPOLOGY)) {pci_d("VIRTIO_BLK topology,physical_block_exp:%x,alignment:%x,min_io_size:%x,opt_io_size:%x\n",p->_config.physical_block_exp,p->_config.alignment_offset,p->_config.min_io_size,p->_config.opt_io_size);}if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_CONFIG_WCE)) {pci_d("VIRTIO_BLK_F_WCE:%x\n",p->_config.wce);}if (virtio_get_guest_feature_bit(to_virtio_dev_t(p), VIRTIO_BLK_F_RO)) {pci_d("VIRTIO_BLK readonly\n");}}
static char buf[512];
struct blk_req {struct blk_outhdr hdr;pthread thread;uchar status;
};
static struct blk_req rq;
static void virtio_blk_read(ulong sector)
{struct addr_size phys[3];rq.hdr.type = VIRTIO_BLK_T_IN;rq.hdr.ioprio = 0;rq.hdr.sector = sector;rq.thread = current;phys[0].vp_addr = V2P((ulong)&rq.hdr);phys[0].vp_size = sizeof(rq.hdr);phys[0].vp_flags = VRING_DESC_F_READ;phys[1].vp_addr = V2P((ulong)buf);phys[1].vp_size = 512;phys[1].vp_flags = VRING_DESC_F_WRITE;uchar status = 0;phys[2].vp_addr = V2P((ulong)&rq.status);phys[2].vp_size = sizeof(status);phys[2].vp_flags = VRING_DESC_F_WRITE;virtio_to_queue(to_virtio_dev_t(&pci_vblk), 0, phys, 3, &rq);suspend_thread();}
static void virtio_blk_check_queue(void)
{struct blk_req *p;size_t len;/* Put the received packets into the recv list */while (virtio_from_queue(to_virtio_dev_t(&pci_vblk),0 , (void **)&p, &len)== 0) {pci_d("virtio_blk_from_queue:%lx,len:%x,status:%d,thread:%s\n", p, len,p->status,p->thread->name);wake_up_thread(p->thread);}}
int init_virtio_blk()
{virtio_dev_t dev = to_virtio_dev_t(&pci_vblk);if (virtio_setup_device(dev, VIRTIO_BLK_DEVICE_ID, 1)) {virtio_blk_read_config(&pci_vblk);//virtio_driver_init(dev);pci_dump_config(to_pci_device_t(&pci_vblk));msi_register_one(to_pci_device_t(&pci_vblk),0,irq_handler);virtio_device_ready(to_virtio_dev_t(&pci_vblk));memset(buf,0x12,512);//set_timeout_nsec(1000000000UL, vblk_timeout,dev);virtio_blk_read(1);dump_mem(buf,512);virtio_blk_read(1);dump_mem(buf,512);}return OK;
}
DECLARE_MODULE(virtio_blk_drivers, 0, main);

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virtio_blk_read发送读取命令之后就suspend_thread挂起当前线程。

在中断处理中如果读取到数据，virtio_blk_check_queue，就恢复挂起的线程。

先把buf内容设置为0x12，然后读取内容，打印内存。

读取2次是测试中断是否正常。即第一次读取之后要通知used_event，否则第二次读取的时候不会发生中断，但是如果去检测队列，是可以读取到内容的。比如在定时器里面去检查队列。

运行结果：

VIRTIO_BLK_F_SIZE_MAX:0
_is_mmio:1,is_64:0,_is_prefetchable:0
VIRTIO_BLK_F_SEG_MAX:7e
addr_size:1000
VIRTIO_BLK cylinders:2,heads:10,sectors:3f
addr_64:febd1000
VIRTIO_BLK_F_BLK_SIZE:200
msix_table_bar:1,val:1,off:44
VIRTIO_BLK topology,physical_block_exp:0,alignment:0,min_io_size:0,opt_io_size:0
[0:4.0] vid:id = 1af4:1000
VIRTIO_BLK_F_WCE:1bar[1]: 32bits addr=000000000000C040 size=20
VIRTIO_BLK readonlybar[2]: 32bits addr=00000000FEBD1000 size=1000
[0:3.0] vid:id = 1af4:1001IRQ = 11bar[1]: 32bits addr=000000000000C000 size=40Have MSI-X!    bar[2]: 32bits addr=00000000FEBD0000 size=1000msix_location: 64    IRQ = 11msix_ctrl: 2    Have MSI-X!        msix_msgnum: 3        msix_location: 64        msix_table_bar: 1        msix_ctrl: 1        msix_table_offset: 0        msix_msgnum: 2        msix_pba_bar: 1        msix_table_bar: 1        msix_pba_offset: 2048msix_table_offset: 0ap start donemsix_pba_bar: 1        msix_pba_offset: 2048
register_irq_handler:100,handler:FFFFFFFF81009120
pci_msix_enable msix_bar:ffffffff810334a0,1
map phy:fea00000 to vaddr:ffffffe8fea00000,&pte:ffffffff80010fa8,pte:fea0009b,511,419,501
ctrl:8001, c001 c001,num:2
msix_mask_entry:0,ffffffe8febd000c,1
msix_mask_entry:1,ffffffe8febd001c,1
msix_mask_entry:0,ffffffe8febd000c,1
msix_write_entry addr:ffffffe8febd0000,msiaddr:fee00000,data:4064
get:fee00000,4064
msix unmask entry :ffffffe8febd000c,1
unmask :0,0,0
free_first:0000000000000000,left:125,dev->threads:0,n:3
vd->addr:10331a0,vd->len:10,vd->flags:1
vd->addr:10331c0,vd->len:200,vd->flags:3
vd->addr:10331b8,vd->len:1,vd->flags:3
cmos:2020-6-5 14:37:3
****virtio_blk_irq_handler**** irq:100,isr:1 cpu:0
virtio_blk_from_queue:ffffffff810331a0,len:201,status:0,thread:kmain
addr:ffffffff810331c0:
~810331c0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810331d0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810331e0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810331f0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033200 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033210 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033220 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033230 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033240 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033250 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033260 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033270 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033280 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033290 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332a0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332b0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332c0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332d0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332e0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332f0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033300 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033310 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033320 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033330 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033340 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033350 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033360 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033370 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033380 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033390 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810333a0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810333b0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
free_first:0000000000000003,left:125,dev->threads:0,n:3
vd->addr:10331a0,vd->len:10,vd->flags:1
vd->addr:10331c0,vd->len:200,vd->flags:3
vd->addr:10331b8,vd->len:1,vd->flags:3
****virtio_blk_irq_handler**** irq:100,isr:1 cpu:0
virtio_blk_from_queue:ffffffff810331a0,len:201,status:0,thread:kmain
addr:ffffffff810331c0:
~810331c0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810331d0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810331e0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810331f0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033200 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033210 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033220 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033230 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033240 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033250 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033260 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033270 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033280 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033290 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332a0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332b0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332c0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332d0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332e0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810332f0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033300 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033310 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033320 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033330 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033340 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033350 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033360 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033370 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033380 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~81033390 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810333a0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
~810333b0 - 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
pstatus:ffffffff81022680,name:slub_mm,status:0
ht:ffffffff810139c0,0
task:ffffffff81022580
alloc 4k:2000
create_thread_oncpu:ffffffff810225d0,stack:ffffffff8003b000,name:tasklet 0
cpu:0,flag:1,state:1
switch first to tasklet ffffffff8003cff8,from:kmain cpu:0
new stack:ffffffff8003cff8,old stack:ffffffff81021ef0
pml4base 102e000  102c000  102b000 511 510 0
thread_main:ffffffff810225d0,ffffffff810139c0,tasklet
smp_thread_main:ffffffff810139c0
flag:9
KHeap: Free:ffffffff81014000,addr,size:2000

可以看到内存打印了2次，内容都是0，那是user.img生成的时候，清0了。

QEMUOPTS =-enable-kvm -cpu host,+x2apic  \-device virtio-blk-pci,id=blk0,drive=hd0,scsi=off \-drive file=./usr.img,if=none,id=hd0,cache=none,aio=native\-netdev type=tap,script=qemu-ifup.sh,id=net0 -device virtio-net-pci,netdev=net0 \-vga vmware -display vnc=192.168.10.2:10\-smp 2 -m 512 $(QEMUEXTRA)yaos.img: $(OUT)/bootblock $(OUT)/kernel.elfdd if=/dev/zero of=yaos.img count=10000dd if=$(OUT)/bootblock of=yaos.img conv=notruncdd if=$(OUT)/kernel.elf of=yaos.img seek=1 conv=notruncusr.img:  $(OUT)/kernel.elfdd if=/dev/zero of=usr.img count=100qemuimg: yaos.img@echo Ctrl+a h for help$(QEMU) -serial mon:stdio  -nographic $(QEMUOPTS) -hda yaos.img
qemu:   out/kernel.elf user.img@echo Ctrl+a h for help$(QEMU) -serial mon:stdio  -nographic $(QEMUOPTS) -kernel out/kernel.elf

QEMUOPTS 做了修改，创建了virtio-blk硬盘，文件为usr.img

运行本例：

git clone https://github.com/saneee/x86_64_kernel.git
cd 0020
make qemu

原文链接“https://zhuanlan.zhihu.com/p/144349599”

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