From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53185) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ehC8s-0003GB-Ih for qemu-devel@nongnu.org; Thu, 01 Feb 2018 05:30:58 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ehC8b-0001E5-QG for qemu-devel@nongnu.org; Thu, 01 Feb 2018 05:30:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38020) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ehC8b-0001Cq-Hh for qemu-devel@nongnu.org; Thu, 01 Feb 2018 05:30:37 -0500 Date: Thu, 1 Feb 2018 10:30:27 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20180201103026.GC2457@work-vm> References: <20180131173154.GA2520@work-vm> <20180201095118.GA2457@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] vhost-user question List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "jack.chen" Cc: qemu , maxime.coquelin@redhat.com * 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=EF=BC=9Fis fd just effective within one pr= ocess? > for example ,if process A open file a.txt,and it gets one fd=E3=80=82th= e 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 = : > > * 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 : > >> > * 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 =3D memory_region_get_fd(mr); > >> >> if (fd > 0) { > >> >> msg.payload.memory.regions[fd_num].userspace_addr =3D > >> >> reg->userspace_addr; > >> >> msg.payload.memory.regions[fd_num].memory_size =3D > >> >> reg->memory_size; > >> >> msg.payload.memory.regions[fd_num].guest_phys_addr =3D > >> >> reg->guest_phys_addr; > >> >> msg.payload.memory.regions[fd_num].mmap_offset =3D of= fset; > >> >> assert(fd_num < VHOST_MEMORY_MAX_NREGIONS); > >> >> fds[fd_num++] =3D fd; > >> >> } > >> >> > >> >> =E2=80=A6=E2=80=A6 > >> >> DPDK code in vhost_user_set_mem_table > >> >> > >> >> mmap_addr =3D mmap(NULL, mmap_size, PROT_READ | PROT_WRITE, > >> >> MAP_SHARED | MAP_POPULATE, fd, 0); > >> >> =E2=80=A6=E2=80=A6 > >> >> > >> >> 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 off= sets. > >> > > >> > 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