Re: [Qemu-devel] vhost-user question

[jack.chen]

Thanks,But my question is how the fd belonging to qemu can be used in
another process such as DPDK？is fd just effective within one process?
for example ,if process A open file a.txt,and it gets one fd。the fd
can only be used in process A ,can it be used in process B?

2018-02-01 17:51 GMT+08:00 Dr. David Alan Gilbert <address@hidden>:
> * jack.chen (address@hidden) wrote:
>> thanks,but I really can not understand how the fd works,can someone
>> explain it or give me some  reference material??
>
> Probably the man page for the mmap system call, or an introduction to
> linux/unix syscalls.  Just remember that any shared memory will be
> 'backed' by a file (or something that works like a file), and if it's a
> file, when you open it you get a file descriptor.  Once you have that fd
> you can map it somewhere else.
>
> Dave
>
>>
>> 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
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK