[Qemu-devel] vhost-user question

[jack.chen]

hello,I am confused when I read vhost-user source code in qemu.I know
vhost-user app shared memory with qemu by mmap,but why it can use fd which
is belong to qemu?
relative code:
 qemu code in function vhost_user_set_mem_table
fd = memory_region_get_fd(mr);
        if (fd > 0) {
            msg.payload.memory.regions[fd_num].userspace_addr =
reg->userspace_addr;
            msg.payload.memory.regions[fd_num].memory_size  =
reg->memory_size;
            msg.payload.memory.regions[fd_num].guest_phys_addr =
reg->guest_phys_addr;
            msg.payload.memory.regions[fd_num].mmap_offset = offset;
            assert(fd_num < VHOST_MEMORY_MAX_NREGIONS);
            fds[fd_num++] = fd;
        }

……
DPDK code in vhost_user_set_mem_table

mmap_addr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_POPULATE, fd, 0);
……

thanks a lot!