mmap PROT_READ | PROT_WRITE

*
 * 用于映射 resource 资源，并获取 PCI BAR
 * @param dev：DPDK 中关于某一个 PCI 设备的抽象实例
 * @param res_id：说明要获取第几个 BAR
 * @param uio_res：用来存放 PCI BAR 资源的结构
 * @param map_idx、uio_res：数组的计数器
*/

int
pci_uio_map_resource_by_index(struct rte_pci_device *dev, int res_idx,
        struct mapped_pci_resource *uio_res, int map_idx)
{
    ..... // 省略
    // 打开 /dev/bus/pci/devices/{pci_addr}/resource0..N 文件
    if (!wc_activate || fd < 0) {
        snprintf(devname, sizeof(devname),
            "%s/" PCI_PRI_FMT "/resource%d",
            rte_pci_get_sysfs_path(),
            loc->domain, loc->bus, loc->devid,
            loc->function, res_idx);

        /* then try to map resource file */
        fd = open(devname, O_RDWR);
        if (fd < 0) {
            RTE_LOG(ERR, EAL, "Cannot open %s: %s
",
                devname, strerror(errno));
            goto error;
        }
    }

    /* try mapping somewhere close to the end of hugepages */
    if (pci_map_addr == NULL)
        pci_map_addr = pci_find_max_end_va();
    // 进行 mmap() 映射，拿到 PCI BAR 在进程虚拟空间下的地址
    mapaddr = pci_map_resource(pci_map_addr, fd, 0,
            (size_t)dev->mem_resource[res_idx].len, 0);
    close(fd);
    if (mapaddr == MAP_FAILED)
        goto error;

    pci_map_addr = RTE_PTR_ADD(mapaddr,
            (size_t)dev->mem_resource[res_idx].len);
        // 将拿到的 PCI BAR 映射至进程虚拟空间内的地址存起来
    maps[map_idx].phaddr = dev->mem_resource[res_idx].phys_addr;
    maps[map_idx].size = dev->mem_resource[res_idx].len;
    maps[map_idx].addr = mapaddr;
    maps[map_idx].offset = 0;
    strcpy(maps[map_idx].path, devname);
    dev->mem_resource[res_idx].addr = mapaddr;

    return 0;

error:
    rte_free(maps[map_idx].path);
    return -1;
}


/*
 * 对 pci/resource0..N 进行 mmap()，将 PCI BAR 空间通过 mmap 的方式映射到进程内部的虚拟空间，供用户态应用来操作设备
*/
void *
pci_map_resource(void *requested_addr, int fd, off_t offset, size_t size,
         int additional_flags)
{
    void *mapaddr;

    // 核心便是这句 mmap，其中要注意的是 offset 必须为 0
    mapaddr = mmap(requested_addr, size, PROT_READ | PROT_WRITE,
            MAP_SHARED | additional_flags, fd, offset);
    if (mapaddr == MAP_FAILED) {
        RTE_LOG(ERR, EAL,
            "%s(): cannot mmap(%d, %p, 0x%zx, 0x%llx): %s (%p)
",
            __func__, fd, requested_addr, size,
            (unsigned long long)offset,
            strerror(errno), mapaddr);
    } else
        RTE_LOG(DEBUG, EAL, "  PCI memory mapped at %p
", mapaddr);

    return mapaddr;
}