Re: [Qemu-devel] vhost-user question

[jack]

   that is what i really  want to know,thanks very very much!!!
   from jack chen
   On 2/1/2018 18:30，[1]Dr. David Alan Gilbert<address@hidden> wrote：

     * jack.chen (address@hidden) wrote:
     > 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 proce
     ss?
     > for example ,if process A open file a.txt,and it gets one fd。the f
     d
     > can only be used in process A ,can it be used in process B?
     Both processes can use the fd - but it needs a trick.
     Unix sockets can pass fd's between processes - so you pass the fd do
     wn
     the socket and it's received on the destination - the number the fd
     gets
     is different on the two processes, but ti still represents the same
     underlying fd on the same file.
     Follow the call to vhost_user_write in vhost_user_set_mem_table you
     see
     it takes the 'fds' array, that eventually gets passed over the unix
     socket.
     Dave
     > 2018-02-01 17:51 GMT+08:00 Dr. David Alan Gilbert <address@hidden
     .com>:
     > > * jack.chen (address@hidden) wrote:
     > >> thanks,but I really can not understand how the fd works,can som
     eone
     > >> explain it or give me some  reference material??
     > >
     > > Probably the man page for the mmap system call, or an introducti
     on 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
     at.com>:
     > >> > * jack.chen (address@hidden) wrote:
     > >> >> hello,I am confused when I read vhost-user source code in qe
     mu.I know
     > >> >> vhost-user app shared memory with qemu by mmap,but why it ca
     n 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_add
     r =
     > >> >> reg->userspace_addr;
     > >> >>             msg.payload.memory.regions[fd_num].memory_size
     =
     > >> >> reg->memory_size;
     > >> >>             msg.payload.memory.regions[fd_num].guest_phys_ad
     dr =
     > >> >> 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 m
     map;
     > >> > each fd corresponds to the backing file of the memory area th
     at's being
     > >> > shared.  This way the dpdk/vhost doesn't need to open those f
     iles 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 an
     d offsets.
     > >> >
     > >> > Dave
     > >> >
     > >> > --
     > >> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
     > > --
     > > Dr. David Alan Gilbert / address@hidden / Manchester, UK
     --
     Dr. David Alan Gilbert / address@hidden / Manchester, UK

References

   1. mailto:address@hidden