Lguest support for Virtio

This makes lguest able to use the virtio devices.

We change the device descriptor page from a simple array to a variable
length "type, config_len, status, config data..." format, and
implement virtio_config_ops to read from that config data.

We use the virtio ring implementation for an efficient Guest <-> Host
virtqueue mechanism, and the new LHCALL_NOTIFY hypercall to kick the
host when it changes.

We also use LHCALL_NOTIFY on kernel addresses for very very early
console output.  We could have another hypercall, but this hack works
quite well.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/lguest/Kconfig

@@ -2,8 +2,13 @@ config LGUEST_GUEST
 	bool "Lguest guest support"
 	select PARAVIRT
 	depends on !X86_PAE
+	select VIRTIO
 	select VIRTIO_RING
+	select VIRTIO_CONSOLE
 	help
 	  Lguest is a tiny in-kernel hypervisor.  Selecting this will
 	  allow your kernel to boot under lguest.  This option will increase
 	  your kernel size by about 6k.  If in doubt, say N.
+
+	  If you say Y here, make sure you say Y (or M) to the virtio block
+	  and net drivers which lguest needs.

arch/x86/lguest/boot.c

@@ -55,6 +55,7 @@
 #include <linux/clockchips.h>
 #include <linux/lguest.h>
 #include <linux/lguest_launcher.h>
+#include <linux/virtio_console.h>
 #include <asm/paravirt.h>
 #include <asm/param.h>
 #include <asm/page.h>
@@ -849,6 +850,23 @@ static __init char *lguest_memory_setup(void)
 	return "LGUEST";
 }
 
+/* Before virtqueues are set up, we use LHCALL_NOTIFY on normal memory to
+ * produce console output. */
+static __init int early_put_chars(u32 vtermno, const char *buf, int count)
+{
+	char scratch[17];
+	unsigned int len = count;
+
+	if (len > sizeof(scratch) - 1)
+		len = sizeof(scratch) - 1;
+	scratch[len] = '\0';
+	memcpy(scratch, buf, len);
+	hcall(LHCALL_NOTIFY, __pa(scratch), 0, 0);
+
+	/* This routine returns the number of bytes actually written. */
+	return len;
+}
+
 /*G:050
  * Patching (Powerfully Placating Performance Pedants)
  *
@@ -1048,6 +1066,9 @@ __init void lguest_init(void *boot)
 	 * adapted for lguest's use. */
 	add_preferred_console("hvc", 0, NULL);
 
+	/* Register our very early console. */
+	virtio_cons_early_init(early_put_chars);
+
 	/* Last of all, we set the power management poweroff hook to point to
 	 * the Guest routine to power off. */
 	pm_power_off = lguest_power_off;

drivers/lguest/Makefile

@@ -1,3 +1,6 @@
+# Guest requires the device configuration and probing code.
+obj-$(CONFIG_LGUEST_GUEST) += lguest_device.o
+
 # Host requires the other files, which can be a module.
 obj-$(CONFIG_LGUEST)	+= lg.o
 lg-y = core.o hypercalls.o page_tables.o interrupts_and_traps.o \