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, 57 insertions, 3 deletions

diff --git a/include/linux/lguest_launcher.h b/include/linux/lguest_launcher.h
index 0ba414a40c80..641670579446 100644
--- a/include/linux/lguest_launcher.h
+++ b/include/linux/lguest_launcher.h
@@ -9,14 +9,45 @@
 /* How many devices?  Assume each one wants up to two dma arrays per device. */
 #define LGUEST_MAX_DEVICES (LGUEST_MAX_DMA/2)
 
+/*D:200
+ * Lguest I/O
+ *
+ * The lguest I/O mechanism is the only way Guests can talk to devices.  There
+ * are two hypercalls involved: SEND_DMA for output and BIND_DMA for input.  In
+ * each case, "struct lguest_dma" describes the buffer: this contains 16
+ * addr/len pairs, and if there are fewer buffer elements the len array is
+ * terminated with a 0.
+ *
+ * I/O is organized by keys: BIND_DMA attaches buffers to a particular key, and
+ * SEND_DMA transfers to buffers bound to particular key.  By convention, keys
+ * correspond to a physical address within the device's page.  This means that
+ * devices will never accidentally end up with the same keys, and allows the
+ * Host use The Futex Trick (as we'll see later in our journey).
+ *
+ * SEND_DMA simply indicates a key to send to, and the physical address of the
+ * "struct lguest_dma" to send.  The Host will write the number of bytes
+ * transferred into the "struct lguest_dma"'s used_len member.
+ *
+ * BIND_DMA indicates a key to bind to, a pointer to an array of "struct
+ * lguest_dma"s ready for receiving, the size of that array, and an interrupt
+ * to trigger when data is received.  The Host will only allow transfers into
+ * buffers with a used_len of zero: it then sets used_len to the number of
+ * bytes transferred and triggers the interrupt for the Guest to process the
+ * new input. */
 struct lguest_dma
 {
-	/* 0 if free to be used, filled by hypervisor. */
+	/* 0 if free to be used, filled by the Host. */
  	u32 used_len;
 	unsigned long addr[LGUEST_MAX_DMA_SECTIONS];
 	u16 len[LGUEST_MAX_DMA_SECTIONS];
 };
+/*:*/
 
+/*D:460 This is the layout of a block device memory page.  The Launcher sets up
+ * the num_sectors initially to tell the Guest the size of the disk.  The Guest
+ * puts the type, sector and length of the request in the first three fields,
+ * then DMAs to the Host.  The Host processes the request, sets up the result,
+ * then DMAs back to the Guest. */
 struct lguest_block_page
 {
 	/* 0 is a read, 1 is a write. */
@@ -28,27 +59,47 @@ struct lguest_block_page
 	u32 num_sectors; /* Disk length = num_sectors * 512 */
 };
 
-/* There is a shared page of these. */
+/*D:520 The network device is basically a memory page where all the Guests on
+ * the network publish their MAC (ethernet) addresses: it's an array of "struct
+ * lguest_net": */
 struct lguest_net
 {
 	/* Simply the mac address (with multicast bit meaning promisc). */
 	unsigned char mac[6];
 };
+/*:*/
 
 /* Where the Host expects the Guest to SEND_DMA console output to. */
 #define LGUEST_CONSOLE_DMA_KEY 0
 
-/* We have a page of these descriptors in the lguest_device page. */
+/*D:010
+ * Drivers
+ *
+ * The Guest needs devices to do anything useful.  Since we don't let it touch
+ * real devices (think of the damage it could do!) we provide virtual devices.
+ * We could emulate a PCI bus with various devices on it, but that is a fairly
+ * complex burden for the Host and suboptimal for the Guest, so we have our own
+ * "lguest" bus and simple drivers.
+ *
+ * Devices are described by an array of LGUEST_MAX_DEVICES of these structs,
+ * placed by the Launcher just above the top of physical memory:
+ */
 struct lguest_device_desc {
+	/* The device type: console, network, disk etc. */
 	u16 type;
 #define LGUEST_DEVICE_T_CONSOLE	1
 #define LGUEST_DEVICE_T_NET	2
 #define LGUEST_DEVICE_T_BLOCK	3
 
+	/* The specific features of this device: these depends on device type
+	 * except for LGUEST_DEVICE_F_RANDOMNESS. */
 	u16 features;
 #define LGUEST_NET_F_NOCSUM		0x4000 /* Don't bother checksumming */
 #define LGUEST_DEVICE_F_RANDOMNESS	0x8000 /* IRQ is fairly random */
 
+	/* This is how the Guest reports status of the device: the Host can set
+	 * LGUEST_DEVICE_S_REMOVED to indicate removal, but the rest are only
+	 * ever manipulated by the Guest, and only ever set. */
 	u16 status;
 /* 256 and above are device specific. */
 #define LGUEST_DEVICE_S_ACKNOWLEDGE	1 /* We have seen device. */
@@ -58,9 +109,12 @@ struct lguest_device_desc {
 #define LGUEST_DEVICE_S_REMOVED_ACK	16 /* Driver has been told. */
 #define LGUEST_DEVICE_S_FAILED		128 /* Something actually failed */
 
+	/* Each device exists somewhere in Guest physical memory, over some
+	 * number of pages. */
 	u16 num_pages;
 	u32 pfn;
 };
+/*:*/
 
 /* Write command first word is a request. */
 enum lguest_req