lguest: documentation III: Drivers

Documentation: The Drivers

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 201 insertions, 17 deletions

diff --git a/drivers/net/lguest_net.c b/drivers/net/lguest_net.c
index 112778652f7d..20df6a848923 100644
--- a/drivers/net/lguest_net.c
+++ b/drivers/net/lguest_net.c
@@ -1,6 +1,13 @@
-/* A simple network driver for lguest.
+/*D:500
+ * The Guest network driver.
  *
- * Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
+ * This is very simple a virtual network driver, and our last Guest driver.
+ * The only trick is that it can talk directly to multiple other recipients
+ * (ie. other Guests on the same network).  It can also be used with only the
+ * Host on the network.
+ :*/
+
+/* Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -28,23 +35,28 @@
 #define MAX_LANS                4
 #define NUM_SKBS                8
 
+/*D:530 The "struct lguestnet_info" contains all the information we need to
+ * know about the network device. */
 struct lguestnet_info
 {
-        /* The shared page(s). */
+        /* The mapped device page(s) (an array of "struct lguest_net"). */
         struct lguest_net *peer;
+        /* The physical address of the device page(s) */
         unsigned long peer_phys;
+        /* The size of the device page(s). */
         unsigned long mapsize;
 
         /* The lguest_device I come from */
         struct lguest_device *lgdev;
 
-        /* My peerid. */
+        /* My peerid (ie. my slot in the array). */
         unsigned int me;
 
-        /* Receive queue. */
+        /* Receive queue: the network packets waiting to be filled. */
         struct sk_buff *skb[NUM_SKBS];
         struct lguest_dma dma[NUM_SKBS];
 };
+/*:*/
 
 /* How many bytes left in this page. */
 static unsigned int rest_of_page(void *data)
@@ -52,39 +64,82 @@ static unsigned int rest_of_page(void *data)
         return PAGE_SIZE - ((unsigned long)data % PAGE_SIZE);
 }
 
-/* Simple convention: offset 4 * peernum. */
+/*D:570 Each peer (ie. Guest or Host) on the network binds their receive
+ * buffers to a different key: we simply use the physical address of the
+ * device's memory page plus the peer number.  The Host insists that all keys
+ * be a multiple of 4, so we multiply the peer number by 4. */
 static unsigned long peer_key(struct lguestnet_info *info, unsigned peernum)
 {
         return info->peer_phys + 4 * peernum;
 }
 
+/* This is the routine which sets up a "struct lguest_dma" to point to a
+ * network packet, similar to req_to_dma() in lguest_blk.c.  The structure of a
+ * "struct sk_buff" has grown complex over the years: it consists of a "head"
+ * linear section pointed to by "skb->data", and possibly an array of
+ * "fragments" in the case of a non-linear packet.
+ *
+ * Our receive buffers don't use fragments at all but outgoing skbs might, so
+ * we handle it. */
 static void skb_to_dma(const struct sk_buff *skb, unsigned int headlen,
                        struct lguest_dma *dma)
 {
         unsigned int i, seg;
 
+        /* First, we put the linear region into the "struct lguest_dma".  Each
+         * entry can't go over a page boundary, so even though all our packets
+         * are 1514 bytes or less, we might need to use two entries here: */
         for (i = seg = 0; i < headlen; seg++, i += rest_of_page(skb->data+i)) {
                 dma->addr[seg] = virt_to_phys(skb->data + i);
                 dma->len[seg] = min((unsigned)(headlen - i),
                                     rest_of_page(skb->data + i));
         }
+
+        /* Now we handle the fragments: at least they're guaranteed not to go
+         * over a page.  skb_shinfo(skb) returns a pointer to the structure
+         * which tells us about the number of fragments and the fragment
+         * array. */
         for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, seg++) {
                 const skb_frag_t *f = &skb_shinfo(skb)->frags[i];
                 /* Should not happen with MTU less than 64k - 2 * PAGE_SIZE. */
                 if (seg == LGUEST_MAX_DMA_SECTIONS) {
+                        /* We will end up sending a truncated packet should
+                         * this ever happen.  Plus, a cool log message! */
                         printk("Woah dude!  Megapacket!\n");
                         break;
                 }
                 dma->addr[seg] = page_to_phys(f->page) + f->page_offset;
                 dma->len[seg] = f->size;
         }
+
+        /* If after all that we didn't use the entire "struct lguest_dma"
+         * array, we terminate it with a 0 length. */
         if (seg < LGUEST_MAX_DMA_SECTIONS)
                 dma->len[seg] = 0;
 }
 
-/* We overload multicast bit to show promiscuous mode. */
+/*
+ * Packet transmission.
+ *
+ * Our packet transmission is a little unusual.  A real network card would just
+ * send out the packet and leave the receivers to decide if they're interested.
+ * Instead, we look through the network device memory page and see if any of
+ * the ethernet addresses match the packet destination, and if so we send it to
+ * that Guest.
+ *
+ * This is made a little more complicated in two cases.  The first case is
+ * broadcast packets: for that we send the packet to all Guests on the network,
+ * one at a time.  The second case is "promiscuous" mode, where a Guest wants
+ * to see all the packets on the network.  We need a way for the Guest to tell
+ * us it wants to see all packets, so it sets the "multicast" bit on its
+ * published MAC address, which is never valid in a real ethernet address.
+ */
 #define PROMISC_BIT             0x01
 
+/* This is the callback which is summoned whenever the network device's
+ * multicast or promiscuous state changes.  If the card is in promiscuous mode,
+ * we advertise that in our ethernet address in the device's memory.  We do the
+ * same if Linux wants any or all multicast traffic.  */
 static void lguestnet_set_multicast(struct net_device *dev)
 {
         struct lguestnet_info *info = netdev_priv(dev);
@@ -95,11 +150,14 @@ static void lguestnet_set_multicast(struct net_device *dev)
                 info->peer[info->me].mac[0] &= ~PROMISC_BIT;
 }
 
+/* A simple test function to see if a peer wants to see all packets.*/
 static int promisc(struct lguestnet_info *info, unsigned int peer)
 {
         return info->peer[peer].mac[0] & PROMISC_BIT;
 }
 
+/* Another simple function to see if a peer's advertised ethernet address
+ * matches a packet's destination ethernet address. */
 static int mac_eq(const unsigned char mac[ETH_ALEN],
                   struct lguestnet_info *info, unsigned int peer)
 {
@@ -109,6 +167,8 @@ static int mac_eq(const unsigned char mac[ETH_ALEN],
         return memcmp(mac+1, info->peer[peer].mac+1, ETH_ALEN-1) == 0;
 }
 
+/* This is the function which actually sends a packet once we've decided a
+ * peer wants it: */
 static void transfer_packet(struct net_device *dev,
                             struct sk_buff *skb,
                             unsigned int peernum)
@@ -116,76 +176,134 @@ static void transfer_packet(struct net_device *dev,
         struct lguestnet_info *info = netdev_priv(dev);
         struct lguest_dma dma;
 
+        /* We use our handy "struct lguest_dma" packing function to prepare
+         * the skb for sending. */
         skb_to_dma(skb, skb_headlen(skb), &dma);
         pr_debug("xfer length %04x (%u)\n", htons(skb->len), skb->len);
 
+        /* This is the actual send call which copies the packet. */
         lguest_send_dma(peer_key(info, peernum), &dma);
+
+        /* Check that the entire packet was transmitted.  If not, it could mean
+         * that the other Guest registered a short receive buffer, but this
+         * driver should never do that.  More likely, the peer is dead. */
         if (dma.used_len != skb->len) {
                 dev->stats.tx_carrier_errors++;
                 pr_debug("Bad xfer to peer %i: %i of %i (dma %p/%i)\n",
                          peernum, dma.used_len, skb->len,
                          (void *)dma.addr[0], dma.len[0]);
         } else {
+                /* On success we update the stats. */
                 dev->stats.tx_bytes += skb->len;
                 dev->stats.tx_packets++;
         }
 }
 
+/* Another helper function to tell is if a slot in the device memory is unused.
+ * Since we always set the Local Assignment bit in the ethernet address, the
+ * first byte can never be 0. */
 static int unused_peer(const struct lguest_net peer[], unsigned int num)
 {
         return peer[num].mac[0] == 0;
 }
 
+/* Finally, here is the routine which handles an outgoing packet.  It's called
+ * "start_xmit" for traditional reasons. */
 static int lguestnet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
         unsigned int i;
         int broadcast;
         struct lguestnet_info *info = netdev_priv(dev);
+        /* Extract the destination ethernet address from the packet. */
         const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
 
         pr_debug("%s: xmit %02x:%02x:%02x:%02x:%02x:%02x\n",
                  dev->name, dest[0],dest[1],dest[2],dest[3],dest[4],dest[5]);
 
+        /* If it's a multicast packet, we broadcast to everyone.  That's not
+         * very efficient, but there are very few applications which actually
+         * use multicast, which is a shame really.
+         *
+         * As etherdevice.h points out: "By definition the broadcast address is
+         * also a multicast address."  So we don't have to test for broadcast
+         * packets separately. */
         broadcast = is_multicast_ether_addr(dest);
+
+        /* Look through all the published ethernet addresses to see if we
+         * should send this packet. */
         for (i = 0; i < info->mapsize/sizeof(struct lguest_net); i++) {
+                /* We don't send to ourselves (we actually can't SEND_DMA to
+                 * ourselves anyway), and don't send to unused slots.*/
                 if (i == info->me || unused_peer(info->peer, i))
                         continue;
 
+                /* If it's broadcast we send it.  If they want every packet we
+                 * send it.  If the destination matches their address we send
+                 * it.  Otherwise we go to the next peer. */
                 if (!broadcast && !promisc(info, i) && !mac_eq(dest, info, i))
                         continue;
 
                 pr_debug("lguestnet %s: sending from %i to %i\n",
                          dev->name, info->me, i);
+                /* Our routine which actually does the transfer. */
                 transfer_packet(dev, skb, i);
         }
+
+        /* An xmit routine is expected to dispose of the packet, so we do. */
         dev_kfree_skb(skb);
+
+        /* As per kernel convention, 0 means success.  This is why I love
+         * networking: even if we never sent to anyone, that's still
+         * success! */
         return 0;
 }
 
-/* Find a new skb to put in this slot in shared mem. */
+/*D:560
+ * Packet receiving.
+ *
+ * First, here's a helper routine which fills one of our array of receive
+ * buffers: */
 static int fill_slot(struct net_device *dev, unsigned int slot)
 {
         struct lguestnet_info *info = netdev_priv(dev);
-        /* Try to create and register a new one. */
+
+        /* We can receive ETH_DATA_LEN (1500) byte packets, plus a standard
+         * ethernet header of ETH_HLEN (14) bytes. */
         info->skb[slot] = netdev_alloc_skb(dev, ETH_HLEN + ETH_DATA_LEN);
         if (!info->skb[slot]) {
                 printk("%s: could not fill slot %i\n", dev->name, slot);
                 return -ENOMEM;
         }
 
+        /* skb_to_dma() is a helper which sets up the "struct lguest_dma" to
+         * point to the data in the skb: we also use it for sending out a
+         * packet. */
         skb_to_dma(info->skb[slot], ETH_HLEN + ETH_DATA_LEN, &info->dma[slot]);
+
+        /* This is a Write Memory Barrier: it ensures that the entry in the
+         * receive buffer array is written *before* we set the "used_len" entry
+         * to 0.  If the Host were looking at the receive buffer array from a
+         * different CPU, it could potentially see "used_len = 0" and not see
+         * the updated receive buffer information.  This would be a horribly
+         * nasty bug, so make sure the compiler and CPU know this has to happen
+         * first. */
         wmb();
-        /* Now we tell hypervisor it can use the slot. */
+        /* Writing 0 to "used_len" tells the Host it can use this receive
+         * buffer now. */
         info->dma[slot].used_len = 0;
         return 0;
 }
 
+/* This is the actual receive routine.  When we receive an interrupt from the
+ * Host to tell us a packet has been delivered, we arrive here: */
 static irqreturn_t lguestnet_rcv(int irq, void *dev_id)
 {
         struct net_device *dev = dev_id;
         struct lguestnet_info *info = netdev_priv(dev);
         unsigned int i, done = 0;
 
+        /* Look through our entire receive array for an entry which has data
+         * in it. */
         for (i = 0; i < ARRAY_SIZE(info->dma); i++) {
                 unsigned int length;
                 struct sk_buff *skb;
@@ -194,10 +312,16 @@ static irqreturn_t lguestnet_rcv(int irq, void *dev_id)
                 if (length == 0)
                         continue;
 
+                /* We've found one!  Remember the skb (we grabbed the length
+                 * above), and immediately refill the slot we've taken it
+                 * from. */
                 done++;
                 skb = info->skb[i];
                 fill_slot(dev, i);
 
+                /* This shouldn't happen: micropackets could be sent by a
+                 * badly-behaved Guest on the network, but the Host will never
+                 * stuff more data in the buffer than the buffer length. */
                 if (length < ETH_HLEN || length > ETH_HLEN + ETH_DATA_LEN) {
                         pr_debug(KERN_WARNING "%s: unbelievable skb len: %i\n",
                                  dev->name, length);
@@ -205,36 +329,72 @@ static irqreturn_t lguestnet_rcv(int irq, void *dev_id)
                         continue;
                 }
 
+                /* skb_put(), what a great function!  I've ranted about this
+                 * function before (http://lkml.org/lkml/1999/9/26/24).  You
+                 * call it after you've added data to the end of an skb (in
+                 * this case, it was the Host which wrote the data). */
                 skb_put(skb, length);
+
+                /* The ethernet header contains a protocol field: we use the
+                 * standard helper to extract it, and place the result in
+                 * skb->protocol.  The helper also sets up skb->pkt_type and
+                 * eats up the ethernet header from the front of the packet. */
                 skb->protocol = eth_type_trans(skb, dev);
-                /* This is a reliable transport. */
+
+                /* If this device doesn't need checksums for sending, we also
+                 * don't need to check the packets when they come in. */
                 if (dev->features & NETIF_F_NO_CSUM)
                         skb->ip_summed = CHECKSUM_UNNECESSARY;
+
+                /* As a last resort for debugging the driver or the lguest I/O
+                 * subsystem, you can uncomment the "#define DEBUG" at the top
+                 * of this file, which turns all the pr_debug() into printk()
+                 * and floods the logs. */
                 pr_debug("Receiving skb proto 0x%04x len %i type %i\n",
                          ntohs(skb->protocol), skb->len, skb->pkt_type);
 
+                /* Update the packet and byte counts (visible from ifconfig,
+                 * and good for debugging). */
                 dev->stats.rx_bytes += skb->len;
                 dev->stats.rx_packets++;
+
+                /* Hand our fresh network packet into the stack's "network
+                 * interface receive" routine.  That will free the packet
+                 * itself when it's finished. */
                 netif_rx(skb);
         }
+
+        /* If we found any packets, we assume the interrupt was for us. */
         return done ? IRQ_HANDLED : IRQ_NONE;
 }
 
+/*D:550 This is where we start: when the device is brought up by dhcpd or
+ * ifconfig.  At this point we advertise our MAC address to the rest of the
+ * network, and register receive buffers ready for incoming packets. */
 static int lguestnet_open(struct net_device *dev)
 {
         int i;
         struct lguestnet_info *info = netdev_priv(dev);
 
-        /* Set up our MAC address */
+        /* Copy our MAC address into the device page, so others on the network
+         * can find us. */
         memcpy(info->peer[info->me].mac, dev->dev_addr, ETH_ALEN);
 
-        /* Turn on promisc mode if needed */
+        /* We might already be in promisc mode (dev->flags & IFF_PROMISC).  Our
+         * set_multicast callback handles this already, so we call it now. */
         lguestnet_set_multicast(dev);
 
+        /* Allocate packets and put them into our "struct lguest_dma" array.
+         * If we fail to allocate all the packets we could still limp along,
+         * but it's a sign of real stress so we should probably give up now. */
         for (i = 0; i < ARRAY_SIZE(info->dma); i++) {
                 if (fill_slot(dev, i) != 0)
                         goto cleanup;
         }
+
+        /* Finally we tell the Host where our array of "struct lguest_dma"
+         * receive buffers is, binding it to the key corresponding to the
+         * device's physical memory plus our peerid. */
         if (lguest_bind_dma(peer_key(info,info->me), info->dma,
                             NUM_SKBS, lgdev_irq(info->lgdev)) != 0)
                 goto cleanup;
@@ -245,22 +405,29 @@ cleanup:
                 dev_kfree_skb(info->skb[i]);
         return -ENOMEM;
 }
+/*:*/
 
+/* The close routine is called when the device is no longer in use: we clean up
+ * elegantly. */
 static int lguestnet_close(struct net_device *dev)
 {
         unsigned int i;
         struct lguestnet_info *info = netdev_priv(dev);
 
-        /* Clear all trace: others might deliver packets, we'll ignore it. */
+        /* Clear all trace of our existence out of the device memory by setting
+         * the slot which held our MAC address to 0 (unused). */
         memset(&info->peer[info->me], 0, sizeof(info->peer[info->me]));
 
-        /* Deregister sg lists. */
+        /* Unregister our array of receive buffers */
         lguest_unbind_dma(peer_key(info, info->me), info->dma);
         for (i = 0; i < ARRAY_SIZE(info->dma); i++)
                 dev_kfree_skb(info->skb[i]);
         return 0;
 }
 
+/*D:510 The network device probe function is basically a standard ethernet
+ * device setup.  It reads the "struct lguest_device_desc" and sets the "struct
+ * net_device".  Oh, the line-by-line excitement!  Let's skip over it. :*/
 static int lguestnet_probe(struct lguest_device *lgdev)
 {
         int err, irqf = IRQF_SHARED;
@@ -290,10 +457,16 @@ static int lguestnet_probe(struct lguest_device *lgdev)
         dev->stop = lguestnet_close;
         dev->hard_start_xmit = lguestnet_start_xmit;
 
-        /* Turning on/off promisc will call dev->set_multicast_list.
-         * We don't actually support multicast yet */
+        /* We don't actually support multicast yet, but turning on/off
+         * promisc also calls dev->set_multicast_list. */
         dev->set_multicast_list = lguestnet_set_multicast;
         SET_NETDEV_DEV(dev, &lgdev->dev);
+
+        /* The network code complains if you have "scatter-gather" capability
+         * if you don't also handle checksums (it seem that would be
+         * "illogical").  So we use a lie of omission and don't tell it that we
+         * can handle scattered packets unless we also don't want checksums,
+         * even though to us they're completely independent. */
         if (desc->features & LGUEST_NET_F_NOCSUM)
                 dev->features = NETIF_F_SG|NETIF_F_NO_CSUM;
 
@@ -325,6 +498,9 @@ static int lguestnet_probe(struct lguest_device *lgdev)
         }
 
         pr_debug("lguestnet: registered device %s\n", dev->name);
+        /* Finally, we put the "struct net_device" in the generic "struct
+         * lguest_device"s private pointer.  Again, it's not necessary, but
+         * makes sure the cool kernel kids don't tease us. */
         lgdev->private = dev;
         return 0;
 
@@ -352,3 +528,11 @@ module_init(lguestnet_init);
 
 MODULE_DESCRIPTION("Lguest network driver");
 MODULE_LICENSE("GPL");
+
+/*D:580
+ * This is the last of the Drivers, and with this we have covered the many and
+ * wonderous and fine (and boring) details of the Guest.
+ *
+ * "make Launcher" beckons, where we answer questions like "Where do Guests
+ * come from?", and "What do you do when someone asks for optimization?"
+ */