[xen] Add basic support for PV-HVM domains

Add basic support for Xen PV-HVM domains (detected via the Xen
platform PCI device with IDs 5853:0001), including support for
accessing configuration via XenStore and enumerating devices via
XenBus.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

3 files changed, 1065 insertions, 0 deletions

diff --git a/src/interface/xen/xenbus.c b/src/interface/xen/xenbus.c
new file mode 100644
index 00000000..7ac41899
--- /dev/null
+++ b/src/interface/xen/xenbus.c
@@ -0,0 +1,393 @@
+/*
+ * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * 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 the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdio.h>
+#include <errno.h>
+#include <ipxe/malloc.h>
+#include <ipxe/device.h>
+#include <ipxe/timer.h>
+#include <ipxe/nap.h>
+#include <ipxe/xen.h>
+#include <ipxe/xenstore.h>
+#include <ipxe/xenbus.h>
+
+/** @file
+ *
+ * Xen device bus
+ *
+ */
+
+/* Disambiguate the various error causes */
+#define ETIMEDOUT_UNKNOWN						\
+	__einfo_error ( EINFO_ETIMEDOUT_UNKNOWN )
+#define EINFO_ETIMEDOUT_UNKNOWN						\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateUnknown,		\
+			  "Unknown" )
+#define ETIMEDOUT_INITIALISING						\
+	__einfo_error ( EINFO_ETIMEDOUT_INITIALISING )
+#define EINFO_ETIMEDOUT_INITIALISING					\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateInitialising,	\
+			  "Initialising" )
+#define ETIMEDOUT_INITWAIT						\
+	__einfo_error ( EINFO_ETIMEDOUT_INITWAIT )
+#define EINFO_ETIMEDOUT_INITWAIT					\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateInitWait,		\
+			  "InitWait" )
+#define ETIMEDOUT_INITIALISED						\
+	__einfo_error ( EINFO_ETIMEDOUT_INITIALISED )
+#define EINFO_ETIMEDOUT_INITIALISED					\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateInitialised,	\
+			  "Initialised" )
+#define ETIMEDOUT_CONNECTED						\
+	__einfo_error ( EINFO_ETIMEDOUT_CONNECTED )
+#define EINFO_ETIMEDOUT_CONNECTED					\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateConnected,	\
+			  "Connected" )
+#define ETIMEDOUT_CLOSING						\
+	__einfo_error ( EINFO_ETIMEDOUT_CLOSING )
+#define EINFO_ETIMEDOUT_CLOSING						\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateClosing,		\
+			  "Closing" )
+#define ETIMEDOUT_CLOSED						\
+	__einfo_error ( EINFO_ETIMEDOUT_CLOSED )
+#define EINFO_ETIMEDOUT_CLOSED						\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateClosed,		\
+			  "Closed" )
+#define ETIMEDOUT_RECONFIGURING						\
+	__einfo_error ( EINFO_ETIMEDOUT_RECONFIGURING )
+#define EINFO_ETIMEDOUT_RECONFIGURING					\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateReconfiguring,	\
+			  "Reconfiguring" )
+#define ETIMEDOUT_RECONFIGURED						\
+	__einfo_error ( EINFO_ETIMEDOUT_RECONFIGURED )
+#define EINFO_ETIMEDOUT_RECONFIGURED					\
+	__einfo_uniqify ( EINFO_ETIMEDOUT, XenbusStateReconfigured,	\
+			  "Reconfigured" )
+#define ETIMEDOUT_STATE( state )					\
+	EUNIQ ( EINFO_ETIMEDOUT, (state), ETIMEDOUT_UNKNOWN,		\
+		ETIMEDOUT_INITIALISING, ETIMEDOUT_INITWAIT,		\
+		ETIMEDOUT_INITIALISED, ETIMEDOUT_CONNECTED,		\
+		ETIMEDOUT_CLOSING, ETIMEDOUT_CLOSED,			\
+		ETIMEDOUT_RECONFIGURING, ETIMEDOUT_RECONFIGURED )
+
+/** Maximum time to wait for backend to reach a given state, in ticks */
+#define XENBUS_BACKEND_TIMEOUT ( 5 * TICKS_PER_SEC )
+
+/**
+ * Set device state
+ *
+ * @v xendev		Xen device
+ * @v state		New state
+ * @ret rc		Return status code
+ */
+int xenbus_set_state ( struct xen_device *xendev, int state ) {
+	int rc;
+
+	/* Attempt to set state */
+	if ( ( rc = xenstore_write_num ( xendev->xen, state, xendev->key,
+					 "state", NULL ) ) != 0 ) {
+		DBGC ( xendev, "XENBUS %s could not set state=\"%d\": %s\n",
+		       xendev->key, state, strerror ( rc ) );
+		return rc;
+	}
+
+	return 0;
+}
+
+/**
+ * Get backend state
+ *
+ * @v xendev		Xen device
+ * @ret state		Backend state, or negative error
+ */
+static int xenbus_backend_state ( struct xen_device *xendev ) {
+	unsigned long state;
+	int rc;
+
+	/* Attempt to get backend state */
+	if ( ( rc = xenstore_read_num ( xendev->xen, &state, xendev->backend,
+					"state", NULL ) ) != 0 ) {
+		DBGC ( xendev, "XENBUS %s could not read %s/state: %s\n",
+		       xendev->key, xendev->backend, strerror ( rc ) );
+		return rc;
+	}
+
+	return state;
+}
+
+/**
+ * Wait for backend to reach a given state
+ *
+ * @v xendev		Xen device
+ * @v state		Desired backend state
+ * @ret rc		Return status code
+ */
+int xenbus_backend_wait ( struct xen_device *xendev, int state ) {
+	unsigned long started = currticks();
+	unsigned long elapsed;
+	unsigned int attempts = 0;
+	int current_state;
+	int rc;
+
+	/* Wait for backend to reach this state */
+	do {
+
+		/* Get current backend state */
+		current_state = xenbus_backend_state ( xendev );
+		if ( current_state < 0 ) {
+			rc = current_state;
+			return rc;
+		}
+		if ( current_state == state )
+			return 0;
+
+		/* Allow time for backend to react */
+		cpu_nap();
+
+		/* XenStore is a very slow interface; any fixed delay
+		 * time would be dwarfed by the XenStore access time.
+		 * We therefore use wall clock to time out this
+		 * operation.
+		 */
+		elapsed = ( currticks() - started );
+		attempts++;
+
+	} while ( elapsed < XENBUS_BACKEND_TIMEOUT );
+
+	/* Construct status code from current backend state */
+	rc = -ETIMEDOUT_STATE ( current_state );
+	DBGC ( xendev, "XENBUS %s timed out after %d attempts waiting for "
+	       "%s/state=\"%d\": %s\n", xendev->key, attempts, xendev->backend,
+	       state, strerror ( rc ) );
+
+	return rc;
+}
+
+/**
+ * Find driver for Xen device
+ *
+ * @v type		Device type
+ * @ret driver		Driver, or NULL
+ */
+static struct xen_driver * xenbus_find_driver ( const char *type ) {
+	struct xen_driver *xendrv;
+
+	for_each_table_entry ( xendrv, XEN_DRIVERS ) {
+		if ( strcmp ( xendrv->type, type ) == 0 )
+			return xendrv;
+	}
+	return NULL;
+}
+
+/**
+ * Probe Xen device
+ *
+ * @v xen		Xen hypervisor
+ * @v parent		Parent device
+ * @v type		Device type
+ * @v instance		Device instance
+ * @ret rc		Return status code
+ */
+static int xenbus_probe_device ( struct xen_hypervisor *xen,
+				 struct device *parent, const char *type,
+				 const char *instance ) {
+	struct xen_device *xendev;
+	size_t key_len;
+	int rc;
+
+	/* Allocate and initialise structure */
+	key_len = ( 7 /* "device/" */ + strlen ( type ) + 1 /* "/" */ +
+		    strlen ( instance ) + 1 /* NUL */ );
+	xendev = zalloc ( sizeof ( *xendev ) + key_len );
+	if ( ! xendev ) {
+		rc = -ENOMEM;
+		goto err_alloc;
+	}
+	snprintf ( xendev->dev.name, sizeof ( xendev->dev.name ), "%s/%s",
+		   type, instance );
+	xendev->dev.desc.bus_type = BUS_TYPE_XEN;
+	INIT_LIST_HEAD ( &xendev->dev.children );
+	list_add_tail ( &xendev->dev.siblings, &parent->children );
+	xendev->dev.parent = parent;
+	xendev->xen = xen;
+	xendev->key = ( ( void * ) ( xendev + 1 ) );
+	snprintf ( xendev->key, key_len, "device/%s/%s", type, instance );
+
+	/* Read backend key */
+	if ( ( rc = xenstore_read ( xen, &xendev->backend, xendev->key,
+				    "backend", NULL ) ) != 0 ) {
+		DBGC ( xendev, "XENBUS %s could not read backend: %s\n",
+		       xendev->key, strerror ( rc ) );
+		goto err_read_backend;
+	}
+
+	/* Read backend domain ID */
+	if ( ( rc = xenstore_read_num ( xen, &xendev->backend_id, xendev->key,
+					"backend-id", NULL ) ) != 0 ) {
+		DBGC ( xendev, "XENBUS %s could not read backend-id: %s\n",
+		       xendev->key, strerror ( rc ) );
+		goto err_read_backend_id;
+	}
+	DBGC ( xendev, "XENBUS %s backend=\"%s\" in domain %ld\n",
+	       xendev->key, xendev->backend, xendev->backend_id );
+
+	/* Look for a driver */
+	xendev->driver = xenbus_find_driver ( type );
+	if ( ! xendev->driver ) {
+		DBGC ( xendev, "XENBUS %s has no driver\n", xendev->key );
+		/* Not a fatal error */
+		rc = 0;
+		goto err_no_driver;
+	}
+	xendev->dev.driver_name = xendev->driver->name;
+	DBGC ( xendev, "XENBUS %s has driver \"%s\"\n", xendev->key,
+	       xendev->driver->name );
+
+	/* Probe driver */
+	if ( ( rc = xendev->driver->probe ( xendev ) ) != 0 ) {
+		DBGC ( xendev, "XENBUS could not probe %s: %s\n",
+		       xendev->key, strerror ( rc ) );
+		goto err_probe;
+	}
+
+	return 0;
+
+	xendev->driver->remove ( xendev );
+ err_probe:
+ err_no_driver:
+ err_read_backend_id:
+	free ( xendev->backend );
+ err_read_backend:
+	list_del ( &xendev->dev.siblings );
+	free ( xendev );
+ err_alloc:
+	return rc;
+}
+
+/**
+ * Remove Xen device
+ *
+ * @v xendev		Xen device
+ */
+static void xenbus_remove_device ( struct xen_device *xendev ) {
+
+	/* Remove device */
+	xendev->driver->remove ( xendev );
+	free ( xendev->backend );
+	list_del ( &xendev->dev.siblings );
+	free ( xendev );
+}
+
+/**
+ * Probe Xen devices of a given type
+ *
+ * @v xen		Xen hypervisor
+ * @v parent		Parent device
+ * @v type		Device type
+ * @ret rc		Return status code
+ */
+static int xenbus_probe_type ( struct xen_hypervisor *xen,
+			       struct device *parent, const char *type ) {
+	char *children;
+	char *child;
+	size_t len;
+	int rc;
+
+	/* Get children of this key */
+	if ( ( rc = xenstore_directory ( xen, &children, &len, "device",
+					 type, NULL ) ) != 0 ) {
+		DBGC ( xen, "XENBUS could not list \"%s\" devices: %s\n",
+		       type, strerror ( rc ) );
+		goto err_directory;
+	}
+
+	/* Probe each child */
+	for ( child = children ; child < ( children + len ) ;
+	      child += ( strlen ( child ) + 1 /* NUL */ ) ) {
+		if ( ( rc = xenbus_probe_device ( xen, parent, type,
+						  child ) ) != 0 )
+			goto err_probe_device;
+	}
+
+	free ( children );
+	return 0;
+
+ err_probe_device:
+	free ( children );
+ err_directory:
+	return rc;
+}
+
+/**
+ * Probe Xen bus
+ *
+ * @v xen		Xen hypervisor
+ * @v parent		Parent device
+ * @ret rc		Return status code
+ */
+int xenbus_probe ( struct xen_hypervisor *xen, struct device *parent ) {
+	char *types;
+	char *type;
+	size_t len;
+	int rc;
+
+	/* Get children of "device" key */
+	if ( ( rc = xenstore_directory ( xen, &types, &len, "device",
+					 NULL ) ) != 0 ) {
+		DBGC ( xen, "XENBUS could not list device types: %s\n",
+		       strerror ( rc ) );
+		goto err_directory;
+	}
+
+	/* Probe each child type */
+	for ( type = types ; type < ( types + len ) ;
+	      type += ( strlen ( type ) + 1 /* NUL */ ) ) {
+		if ( ( rc = xenbus_probe_type ( xen, parent, type ) ) != 0 )
+			goto err_probe_type;
+	}
+
+	free ( types );
+	return 0;
+
+	xenbus_remove ( xen, parent );
+ err_probe_type:
+	free ( types );
+ err_directory:
+	return rc;
+}
+
+/**
+ * Remove Xen bus
+ *
+ * @v xen		Xen hypervisor
+ * @v parent		Parent device
+ */
+void xenbus_remove ( struct xen_hypervisor *xen __unused,
+		     struct device *parent ) {
+	struct xen_device *xendev;
+	struct xen_device *tmp;
+
+	/* Remove devices */
+	list_for_each_entry_safe ( xendev, tmp, &parent->children,
+				   dev.siblings ) {
+		xenbus_remove_device ( xendev );
+	}
+}
diff --git a/src/interface/xen/xengrant.c b/src/interface/xen/xengrant.c
new file mode 100644
index 00000000..55731bfa
--- /dev/null
+++ b/src/interface/xen/xengrant.c
@@ -0,0 +1,125 @@
+/*
+ * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * 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 the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+#include <errno.h>
+#include <assert.h>
+#include <ipxe/io.h>
+#include <ipxe/xen.h>
+#include <ipxe/xengrant.h>
+
+/** @file
+ *
+ * Xen grant tables
+ *
+ */
+
+/**
+ * Allocate grant references
+ *
+ * @v xen		Xen hypervisor
+ * @v refs		Grant references to fill in
+ * @v count		Number of references
+ * @ret rc		Return status code
+ */
+int xengrant_alloc ( struct xen_hypervisor *xen, grant_ref_t *refs,
+		     unsigned int count ) {
+	union grant_entry_v2 *entry;
+	unsigned int mask = ( xen->grant.count - 1 );
+	unsigned int check = 0;
+	unsigned int avail;
+	unsigned int ref;
+
+	/* Fail unless we have enough references available */
+	avail = ( xen->grant.count - xen->grant.used -
+		  GNTTAB_NR_RESERVED_ENTRIES );
+	if ( avail < count ) {
+		DBGC ( xen, "XENGRANT cannot allocate %d references (only %d "
+		       "of %d available)\n", count, avail, xen->grant.count );
+		return -ENOBUFS;
+	}
+	DBGC ( xen, "XENGRANT allocating %d references (from %d of %d "
+	       "available)\n", count, avail, xen->grant.count );
+
+	/* Update number of references used */
+	xen->grant.used += count;
+
+	/* Find unused references */
+	for ( ref = xen->grant.ref ; count ; ref = ( ( ref + 1 ) & mask ) ) {
+
+		/* Sanity check */
+		assert ( check++ < xen->grant.count );
+
+		/* Skip reserved references */
+		if ( ref < GNTTAB_NR_RESERVED_ENTRIES )
+			continue;
+
+		/* Skip in-use references */
+		entry = &xen->grant.table[ref];
+		if ( readw ( &entry->hdr.flags ) & GTF_type_mask )
+			continue;
+		if ( readw ( &entry->hdr.domid ) == DOMID_SELF )
+			continue;
+
+		/* Mark reference as in-use.  We leave the flags as
+		 * empty (to avoid creating a valid grant table entry)
+		 * and set the domid to DOMID_SELF.
+		 */
+		writew ( DOMID_SELF, &entry->hdr.domid );
+		DBGC2 ( xen, "XENGRANT allocated ref %d\n", ref );
+
+		/* Record reference */
+		refs[--count] = ref;
+	}
+
+	/* Update cursor */
+	xen->grant.ref = ref;
+
+	return 0;
+}
+
+/**
+ * Free grant references
+ *
+ * @v xen		Xen hypervisor
+ * @v refs		Grant references
+ * @v count		Number of references
+ */
+void xengrant_free ( struct xen_hypervisor *xen, grant_ref_t *refs,
+		     unsigned int count ) {
+	union grant_entry_v2 *entry;
+	unsigned int ref;
+	unsigned int i;
+
+	/* Free references */
+	for ( i = 0 ; i < count ; i++ ) {
+
+		/* Sanity check */
+		ref = refs[i];
+		assert ( ref < xen->grant.count );
+
+		/* Mark reference as unused */
+		entry = &xen->grant.table[ref];
+		writew ( 0, &entry->hdr.flags );
+		writew ( 0, &entry->hdr.domid );
+		DBGC2 ( xen, "XENGRANT freed ref %d\n", ref );
+	}
+}
diff --git a/src/interface/xen/xenstore.c b/src/interface/xen/xenstore.c
new file mode 100644
index 00000000..b9698292
--- /dev/null
+++ b/src/interface/xen/xenstore.c
@@ -0,0 +1,547 @@
+/*
+ * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * 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 the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+#include <stdarg.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <ipxe/io.h>
+#include <ipxe/nap.h>
+#include <ipxe/malloc.h>
+#include <ipxe/xen.h>
+#include <ipxe/xenevent.h>
+#include <ipxe/xenstore.h>
+
+/*
+ * xs_wire.h attempts to define a static error table xsd_errors, which
+ * interacts badly with the dynamically generated error numbers used
+ * by iPXE.  Prevent this table from being constructed by including
+ * errno.h only after including xs_wire.h.
+ *
+ */
+#include <xen/io/xs_wire.h>
+#include <errno.h>
+
+/** @file
+ *
+ * XenStore interface
+ *
+ */
+
+/** Request identifier */
+static uint32_t xenstore_req_id;
+
+/**
+ * Send XenStore request raw data
+ *
+ * @v xen		Xen hypervisor
+ * @v data		Data buffer
+ * @v len		Length of data
+ */
+static void xenstore_send ( struct xen_hypervisor *xen, const void *data,
+			    size_t len ) {
+	struct xenstore_domain_interface *intf = xen->store.intf;
+	XENSTORE_RING_IDX prod = readl ( &intf->req_prod );
+	XENSTORE_RING_IDX cons;
+	XENSTORE_RING_IDX idx;
+	const char *bytes = data;
+	size_t offset = 0;
+	size_t fill;
+
+	DBGCP ( intf, "XENSTORE raw request:\n" );
+	DBGCP_HDA ( intf, MASK_XENSTORE_IDX ( prod ), data, len );
+
+	/* Write one byte at a time */
+	while ( offset < len ) {
+
+		/* Wait for space to become available */
+		while ( 1 ) {
+			cons = readl ( &intf->req_cons );
+			fill = ( prod - cons );
+			if ( fill < XENSTORE_RING_SIZE )
+				break;
+			DBGC2 ( xen, "." );
+			cpu_nap();
+			rmb();
+		}
+
+		/* Write byte */
+		idx = MASK_XENSTORE_IDX ( prod++ );
+		writeb ( bytes[offset++], &intf->req[idx] );
+	}
+
+	/* Update producer counter */
+	wmb();
+	writel ( prod, &intf->req_prod );
+	wmb();
+}
+
+/**
+ * Send XenStore request string (excluding terminating NUL)
+ *
+ * @v xen		Xen hypervisor
+ * @v string		String
+ */
+static void xenstore_send_string ( struct xen_hypervisor *xen,
+				   const char *string ) {
+
+	xenstore_send ( xen, string, strlen ( string ) );
+}
+
+/**
+ * Receive XenStore response raw data
+ *
+ * @v xen		Xen hypervisor
+ * @v data		Data buffer, or NULL to discard data
+ * @v len		Length of data
+ */
+static void xenstore_recv ( struct xen_hypervisor *xen, void *data,
+			    size_t len ) {
+	struct xenstore_domain_interface *intf = xen->store.intf;
+	XENSTORE_RING_IDX cons = readl ( &intf->rsp_cons );
+	XENSTORE_RING_IDX prod;
+	XENSTORE_RING_IDX idx;
+	char *bytes = data;
+	size_t offset = 0;
+	size_t fill;
+
+	DBGCP ( intf, "XENSTORE raw response:\n" );
+
+	/* Read one byte at a time */
+	while ( offset < len ) {
+
+		/* Wait for data to be ready */
+		while ( 1 ) {
+			prod = readl ( &intf->rsp_prod );
+			fill = ( prod - cons );
+			if ( fill > 0 )
+				break;
+			DBGC2 ( xen, "." );
+			cpu_nap();
+			rmb();
+		}
+
+		/* Read byte */
+		idx = MASK_XENSTORE_IDX ( cons++ );
+		if ( data )
+			bytes[offset++] = readb ( &intf->rsp[idx] );
+	}
+	if ( data )
+		DBGCP_HDA ( intf, MASK_XENSTORE_IDX ( cons - len ), data, len );
+
+	/* Update consumer counter */
+	writel ( cons, &intf->rsp_cons );
+	wmb();
+}
+
+/**
+ * Send XenStore request
+ *
+ * @v xen		Xen hypervisor
+ * @v type		Message type
+ * @v req_id		Request ID
+ * @v value		Value, or NULL to omit
+ * @v key		Key path components
+ * @ret rc		Return status code
+ */
+static int xenstore_request ( struct xen_hypervisor *xen,
+			      enum xsd_sockmsg_type type, uint32_t req_id,
+			      const char *value, va_list key ) {
+	struct xsd_sockmsg msg;
+	struct evtchn_send event;
+	const char *string;
+	va_list tmp;
+	int xenrc;
+	int rc;
+
+	/* Construct message header */
+	msg.type = type;
+	msg.req_id = req_id;
+	msg.tx_id = 0;
+	msg.len = 0;
+	DBGC2 ( xen, "XENSTORE request ID %d type %d ", req_id, type );
+
+	/* Calculate total length */
+	va_copy ( tmp, key );
+	while ( ( string = va_arg ( tmp, const char * ) ) != NULL ) {
+		DBGC2 ( xen, "%s%s", ( msg.len ? "/" : "" ), string );
+		msg.len += ( strlen ( string ) + 1 /* '/' or NUL */ );
+	}
+	va_end ( tmp );
+	if ( value ) {
+		DBGC2 ( xen, " = \"%s\"", value );
+		msg.len += strlen ( value );
+	}
+	DBGC2 ( xen, "\n" );
+
+	/* Send message */
+	xenstore_send ( xen, &msg, sizeof ( msg ) );
+	string = va_arg ( key, const char * );
+	assert ( string != NULL );
+	xenstore_send_string ( xen, string );
+	while ( ( string = va_arg ( key, const char * ) ) != NULL ) {
+		xenstore_send_string ( xen, "/" );
+		xenstore_send_string ( xen, string );
+	}
+	xenstore_send ( xen, "", 1 ); /* Separating NUL */
+	if ( value )
+		xenstore_send_string ( xen, value );
+
+	/* Notify the back end */
+	event.port = xen->store.port;
+	if ( ( xenrc = xenevent_send ( xen, &event ) ) != 0 ) {
+		rc = -EXEN ( xenrc );
+		DBGC ( xen, "XENSTORE could not notify back end: %s\n",
+		       strerror ( rc ) );
+		return rc;
+	}
+
+	return 0;
+}
+
+/**
+ * Receive XenStore response
+ *
+ * @v xen		Xen hypervisor
+ * @v req_id		Request ID
+ * @v value		Value to fill in
+ * @v len		Length to fill in
+ * @ret rc		Return status code
+ *
+ * The caller is responsible for eventually calling free() on the
+ * returned value.  Note that the value may comprise multiple
+ * NUL-terminated strings concatenated together.  A terminating NUL
+ * will always be appended to the returned value.
+ */
+static int xenstore_response ( struct xen_hypervisor *xen, uint32_t req_id,
+			       char **value, size_t *len ) {
+	struct xsd_sockmsg msg;
+	char *string;
+	int rc;
+
+	/* Receive message header */
+	xenstore_recv ( xen, &msg, sizeof ( msg ) );
+	*len = msg.len;
+
+	/* Allocate space for response */
+	*value = zalloc ( msg.len + 1 /* terminating NUL */ );
+
+	/* Receive data.  Do this even if allocation failed, or if the
+	 * request ID was incorrect, to avoid leaving data in the
+	 * ring.
+	 */
+	xenstore_recv ( xen, *value, msg.len );
+
+	/* Validate request ID */
+	if ( msg.req_id != req_id ) {
+		DBGC ( xen, "XENSTORE response ID mismatch (got %d, expected "
+		       "%d)\n", msg.req_id, req_id );
+		rc = -EPROTO;
+		goto err_req_id;
+	}
+
+	/* Check for allocation failure */
+	if ( ! *value ) {
+		DBGC ( xen, "XENSTORE could not allocate %d bytes for "
+		       "response\n", msg.len );
+		rc = -ENOMEM;
+		goto err_alloc;
+	}
+
+	/* Check for explicit errors */
+	if ( msg.type == XS_ERROR ) {
+		DBGC ( xen, "XENSTORE response error \"%s\"\n", *value );
+		rc = -EIO;
+		goto err_explicit;
+	}
+
+	DBGC2 ( xen, "XENSTORE response ID %d\n", req_id );
+	if ( DBG_EXTRA ) {
+		for ( string = *value ; string < ( *value + msg.len ) ;
+		      string += ( strlen ( string ) + 1 /* NUL */ ) ) {
+			DBGC2 ( xen, " - \"%s\"\n", string );
+		}
+	}
+	return 0;
+
+ err_explicit:
+ err_alloc:
+ err_req_id:
+	free ( *value );
+	*value = NULL;
+	return rc;
+}
+
+/**
+ * Issue a XenStore message
+ *
+ * @v xen		Xen hypervisor
+ * @v type		Message type
+ * @v response		Response value to fill in, or NULL to discard
+ * @v len		Response length to fill in, or NULL to ignore
+ * @v request		Request value, or NULL to omit
+ * @v key		Key path components
+ * @ret rc		Return status code
+ */
+static int xenstore_message ( struct xen_hypervisor *xen,
+			      enum xsd_sockmsg_type type, char **response,
+			      size_t *len, const char *request, va_list key ) {
+	char *response_value;
+	size_t response_len;
+	int rc;
+
+	/* Send request */
+	if ( ( rc = xenstore_request ( xen, type, ++xenstore_req_id,
+				       request, key ) ) != 0 )
+		return rc;
+
+	/* Receive response */
+	if ( ( rc = xenstore_response ( xen, xenstore_req_id, &response_value,
+					&response_len ) ) != 0 )
+		return rc;
+
+	/* Return response, if applicable */
+	if ( response ) {
+		*response = response_value;
+	} else {
+		free ( response_value );
+	}
+	if ( len )
+		*len = response_len;
+
+	return 0;
+}
+
+/**
+ * Read XenStore value
+ *
+ * @v xen		Xen hypervisor
+ * @v value		Value to fill in
+ * @v key		Key path components
+ * @ret rc		Return status code
+ *
+ * On a successful return, the caller is responsible for calling
+ * free() on the returned value.
+ */
+static int xenstore_vread ( struct xen_hypervisor *xen, char **value,
+			    va_list key ) {
+
+	return xenstore_message ( xen, XS_READ, value, NULL, NULL, key );
+}
+
+/**
+ * Read XenStore value
+ *
+ * @v xen		Xen hypervisor
+ * @v value		Value to fill in
+ * @v ...		Key path components
+ * @ret rc		Return status code
+ *
+ * On a successful return, the caller is responsible for calling
+ * free() on the returned value.
+ */
+__attribute__ (( sentinel )) int
+xenstore_read ( struct xen_hypervisor *xen, char **value, ... ) {
+	va_list key;
+	int rc;
+
+	va_start ( key, value );
+	rc = xenstore_vread ( xen, value, key );
+	va_end ( key );
+	return rc;
+}
+
+/**
+ * Read XenStore numeric value
+ *
+ * @v xen		Xen hypervisor
+ * @v num		Numeric value to fill in
+ * @v ...		Key path components
+ * @ret rc		Return status code
+ */
+__attribute__ (( sentinel )) int
+xenstore_read_num ( struct xen_hypervisor *xen, unsigned long *num, ... ) {
+	va_list key;
+	char *value;
+	char *endp;
+	int rc;
+
+	/* Try to read text value */
+	va_start ( key, num );
+	rc = xenstore_vread ( xen, &value, key );
+	va_end ( key );
+	if ( rc != 0 )
+		goto err_read;
+
+	/* Try to parse as numeric value */
+	*num = strtoul ( value, &endp, 10 );
+	if ( ( *value == '\0' ) || ( *endp != '\0' ) ) {
+		DBGC ( xen, "XENSTORE found invalid numeric value \"%s\"\n",
+		       value );
+		rc = -EINVAL;
+		goto err_strtoul;
+	}
+
+ err_strtoul:
+	free ( value );
+ err_read:
+	return rc;
+}
+
+/**
+ * Write XenStore value
+ *
+ * @v xen		Xen hypervisor
+ * @v value		Value
+ * @v key		Key path components
+ * @ret rc		Return status code
+ */
+static int xenstore_vwrite ( struct xen_hypervisor *xen, const char *value,
+			     va_list key ) {
+
+	return xenstore_message ( xen, XS_WRITE, NULL, NULL, value, key );
+}
+
+/**
+ * Write XenStore value
+ *
+ * @v xen		Xen hypervisor
+ * @v value		Value
+ * @v ...		Key path components
+ * @ret rc		Return status code
+ */
+__attribute__ (( sentinel )) int
+xenstore_write ( struct xen_hypervisor *xen, const char *value, ... ) {
+	va_list key;
+	int rc;
+
+	va_start ( key, value );
+	rc = xenstore_vwrite ( xen, value, key );
+	va_end ( key );
+	return rc;
+}
+
+/**
+ * Write XenStore numeric value
+ *
+ * @v xen		Xen hypervisor
+ * @v num		Numeric value
+ * @v ...		Key path components
+ * @ret rc		Return status code
+ */
+__attribute__ (( sentinel )) int
+xenstore_write_num ( struct xen_hypervisor *xen, unsigned long num, ... ) {
+	char value[ 21 /* "18446744073709551615" + NUL */ ];
+	va_list key;
+	int rc;
+
+	/* Construct value */
+	snprintf ( value, sizeof ( value ), "%ld", num );
+
+	/* Write value */
+	va_start ( key, num );
+	rc = xenstore_vwrite ( xen, value, key );
+	va_end ( key );
+	return rc;
+}
+
+/**
+ * Delete XenStore value
+ *
+ * @v xen		Xen hypervisor
+ * @v ...		Key path components
+ * @ret rc		Return status code
+ */
+__attribute__ (( sentinel )) int
+xenstore_rm ( struct xen_hypervisor *xen, ... ) {
+	va_list key;
+	int rc;
+
+	va_start ( key, xen );
+	rc = xenstore_message ( xen, XS_RM, NULL, NULL, NULL, key );
+	va_end ( key );
+	return rc;
+}
+
+/**
+ * Read XenStore directory
+ *
+ * @v xen		Xen hypervisor
+ * @v children		Child key names to fill in
+ * @v len		Length of child key names to fill in
+ * @v ...		Key path components
+ * @ret rc		Return status code
+ */
+__attribute__ (( sentinel )) int
+xenstore_directory ( struct xen_hypervisor *xen, char **children, size_t *len,
+		     ... ) {
+	va_list key;
+	int rc;
+
+	va_start ( key, len );
+	rc = xenstore_message ( xen, XS_DIRECTORY, children, len, NULL, key );
+	va_end ( key );
+	return rc;
+}
+
+/**
+ * Dump XenStore directory contents (for debugging)
+ *
+ * @v xen		Xen hypervisor
+ * @v key		Key
+ */
+void xenstore_dump ( struct xen_hypervisor *xen, const char *key ) {
+	char *value;
+	char *children;
+	char *child;
+	char *child_key;
+	size_t len;
+	int rc;
+
+	/* Try to dump current key as a value */
+	if ( ( rc = xenstore_read ( xen, &value, key, NULL ) ) == 0 ) {
+		DBGC ( xen, "%s = \"%s\"\n", key, value );
+		free ( value );
+	}
+
+	/* Try to recurse into each child in turn */
+	if ( ( rc = xenstore_directory ( xen, &children, &len, key,
+					 NULL ) ) == 0 ) {
+		for ( child = children ; child < ( children + len ) ;
+		      child += ( strlen ( child ) + 1 /* NUL */ ) ) {
+
+			/* Construct child key */
+			asprintf ( &child_key, "%s/%s", key, child );
+			if ( ! child_key ) {
+				DBGC ( xen, "XENSTORE could not allocate child "
+				       "key \"%s/%s\"\n", key, child );
+				rc = -ENOMEM;
+				break;
+			}
+
+			/* Recurse into child key, continuing on error */
+			xenstore_dump ( xen, child_key );
+			free ( child_key );
+		}
+		free ( children );
+	}
+}