Re: [Qemu-devel] vhost-user question

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* jack.chen (zhunxun@gmail.com) 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 <dgilbert@redhat.com>:
> > * jack.chen (zhunxun@gmail.com) 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 / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK