Re: [Qemu-devel] vhost-user question

[jack.chen]

thanks,but I really can not understand how the fd works,can someone
explain it or give me some  reference material??


2018-02-01 1:31 GMT+08:00 Dr. David Alan Gilbert <address@hidden>:
> * jack.chen (address@hidden) wrote:
>> 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!
>
> Because that's how the dpdk/vhost-user binary knows what to mmap;
> each fd corresponds to the backing file of the memory area that's being
> shared.  This way the dpdk/vhost doesn't need to open those files itself
> or try and match the exact memory configuration of qemu; QEMU just gives
> it the exact thing it needs to mmap - which is just the fd and offsets.
>
> Dave
>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK