[Qemu-devel] vhost-user question

[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!

Re: [Qemu-devel] vhost-user question

[Dr. David Alan Gilbert]

* 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

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 <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

Re: [Qemu-devel] vhost-user question

[Dr. David Alan Gilbert]

* 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

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 <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

Re: [Qemu-devel] vhost-user question

[Dr. David Alan Gilbert]

* jack.chen (zhunxun@gmail.com) 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 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?

Both processes can use the fd - but it needs a trick.
Unix sockets can pass fd's between processes - so you pass the fd down
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 <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
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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<dgilbert@redhat.com> wrote：

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

References

   1. mailto:dgilbert@redhat.com