1 00:00:00,320 --> 00:00:05,420 The first example that we'll look at is how control and data transfer is 2 00:00:05,420 --> 00:00:12,850 affected in zen for a network virtualization. Each guest has two I/O rings, 3 00:00:12,850 --> 00:00:18,110 one for transmission and one for reception. If the guest wants to transmit 4 00:00:18,110 --> 00:00:22,275 packets, it enqueues descriptors in the transmit 5 00:00:22,275 --> 00:00:25,990 I/O ring. And placing these descriptors in 6 00:00:25,990 --> 00:00:30,480 this IO ring is via hyper [UNKNOWN] that then 7 00:00:30,480 --> 00:00:35,140 provide for desktop systems. The packets that need to be 8 00:00:35,140 --> 00:00:40,460 transmitted are not copies into zen, but the buffers that contain these 9 00:00:40,460 --> 00:00:46,130 packets are in the guest operating system buffers. And what the guest 10 00:00:46,130 --> 00:00:51,670 does is it embeds pointers to these buffers 11 00:00:51,670 --> 00:00:57,670 in the descriptors that have been enqueued for transmission by hypervisor. So in 12 00:00:57,670 --> 00:01:00,580 other words, there's no copying of 13 00:01:00,580 --> 00:01:03,400 the package themselves From the guest operating 14 00:01:03,400 --> 00:01:06,140 system buffers into [INAUDIBLE]. Because the 15 00:01:06,140 --> 00:01:10,260 pointers to the guest operating system buffers 16 00:01:10,260 --> 00:01:13,390 have been embedded in these descriptors. 17 00:01:13,390 --> 00:01:16,690 And for the duration of the transmission, 18 00:01:16,690 --> 00:01:19,560 the pages associated with these network packets 19 00:01:19,560 --> 00:01:22,670 are pinned So that Xen can complete 20 00:01:22,670 --> 00:01:25,000 the transmission of the packets. Now, this 21 00:01:25,000 --> 00:01:30,020 is showing interaction of one guest operating system 22 00:01:30,020 --> 00:01:36,790 with Xen. Of course, more than one VM may want to transmit. And what Xen does 23 00:01:36,790 --> 00:01:42,530 is it adopts its own draw bin packet scheduler. In order to transmit 24 00:01:42,530 --> 00:01:47,510 packets from different virtual machines. Receiving 25 00:01:47,510 --> 00:01:49,570 packets from the network and passing 26 00:01:49,570 --> 00:01:55,820 it to the appropriate domain works exactly similar to what I described for 27 00:01:55,820 --> 00:02:01,260 transmission except in the opposite direction. What xen does when it receives a 28 00:02:01,260 --> 00:02:03,610 network packet intended for a particular 29 00:02:03,610 --> 00:02:08,050 guest, it exchanges the received packet for 30 00:02:08,050 --> 00:02:15,090 one of the guest operating system page that has been already provided to xen as 31 00:02:15,090 --> 00:02:20,050 the holding place for incoming packets. In other words, in order to make things 32 00:02:20,050 --> 00:02:22,660 efficient What a guest operating system will 33 00:02:22,660 --> 00:02:27,000 do is pre-allocate network buffers which are 34 00:02:27,000 --> 00:02:33,110 pages owned by the guest operating system. So when a package comes in, Xen can 35 00:02:33,110 --> 00:02:35,960 directly put the network package into the 36 00:02:35,960 --> 00:02:38,860 buffer that is owned by the guest operating 37 00:02:38,860 --> 00:02:43,010 system. And in queue, a descriptor for 38 00:02:43,010 --> 00:02:46,120 that particular guest operating system. So once again, 39 00:02:46,120 --> 00:02:52,870 we can avoid copying in the direction of going from xen to the guest operating 40 00:02:52,870 --> 00:02:58,770 system. So one of the cute tricks that xen also plays, is that when it receives 41 00:02:58,770 --> 00:03:02,090 a packet into a machine page. Then it 42 00:03:02,090 --> 00:03:05,550 can simply exchange that machine page with some 43 00:03:05,550 --> 00:03:07,580 other page that belongs to the guest operating 44 00:03:07,580 --> 00:03:10,350 system. And there again, is another trick to avoid 45 00:03:10,350 --> 00:03:13,690 copying. Either the guest can pre-allocate a buffer, 46 00:03:13,690 --> 00:03:17,270 in which case, xen can directly put that packet 47 00:03:17,270 --> 00:03:20,890 into the pre-allocated buffer. Or if xen receives 48 00:03:20,890 --> 00:03:24,410 a packet into a machine page, it can simply 49 00:03:24,410 --> 00:03:29,530 swap that machine page. For a page that the guest already owns so those 50 00:03:29,530 --> 00:03:32,540 are two different techinques for avoiding copying 51 00:03:32,540 --> 00:03:32,540 altogether in the reverse direction as well.