Subject: [PATCH 1/7] shm-signal: shared-memory signals

From: Gregory Haskins <[email-masked]>
Date: Mon, 03 Aug 2009 13:17:35 -0400

shm-signal provides a generic shared-memory based bidirectional
signaling mechanism.  It is used in conjunction with an existing
signal transport (such as posix-signals, interrupts, pipes, etc) to
increase the efficiency of the transport since the state information
is directly accessible to both sides of the link.  The shared-memory
design provides very cheap access to features such as event-masking
and spurious delivery mititgation, and is useful implementing higher
level shared-memory constructs such as rings.

We will use this mechanism as the basis for a shared-memory interface
later in the series.

Signed-off-by: Gregory Haskins <[email-masked]>
---

 include/linux/Kbuild       |    1 
 include/linux/shm_signal.h |  189 +++++++++++++++++++++++++++++++++++++++++++
 lib/Kconfig                |    9 ++
 lib/Makefile               |    1 
 lib/shm_signal.c           |  192 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 392 insertions(+), 0 deletions(-)
 create mode 100644 include/linux/shm_signal.h
 create mode 100644 lib/shm_signal.c

diff --git a/include/linux/Kbuild b/include/linux/Kbuild
index 334a359..01d67b6 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -331,6 +331,7 @@ unifdef-y += serial_core.h
 unifdef-y += serial.h
 unifdef-y += serio.h
 unifdef-y += shm.h
+unifdef-y += shm_signal.h
 unifdef-y += signal.h
 unifdef-y += smb_fs.h
 unifdef-y += smb.h
diff --git a/include/linux/shm_signal.h b/include/linux/shm_signal.h
new file mode 100644
index 0000000..21cf750
--- /dev/null
+++ b/include/linux/shm_signal.h
@@ -0,0 +1,189 @@
+/*
+ * Copyright 2009 Novell.  All Rights Reserved.
+ *
+ * Author:
+ *      Gregory Haskins <[email-masked]>
+ *
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License
+ * as published by the Free Software Foundation.
+ *
+ * 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 St, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#ifndef _LINUX_SHM_SIGNAL_H
+#define _LINUX_SHM_SIGNAL_H
+
+#include <linux/types.h>
+
+/*
+ *---------
+ * The following structures represent data that is shared across boundaries
+ * which may be quite disparate from one another (e.g. Windows vs Linux,
+ * 32 vs 64 bit, etc).  Therefore, care has been taken to make sure they
+ * present data in a manner that is independent of the environment.
+ *-----------
+ */
+
+#define SHM_SIGNAL_MAGIC 0x58fa39df
+#define SHM_SIGNAL_VER   1
+
+struct shm_signal_irq {
+	__u8                  enabled;
+	__u8                  pending;
+	__u8                  dirty;
+};
+
+enum shm_signal_locality {
+	shm_locality_north,
+	shm_locality_south,
+};
+
+struct shm_signal_desc {
+	__u32                 magic;
+	__u32                 ver;
+	struct shm_signal_irq irq[2];
+};
+
+/* --- END SHARED STRUCTURES --- */
+
+#ifdef __KERNEL__
+
+#include <linux/kref.h>
+#include <linux/interrupt.h>
+
+struct shm_signal_notifier {
+	void (*signal)(struct shm_signal_notifier *);
+};
+
+struct shm_signal;
+
+struct shm_signal_ops {
+	int      (*inject)(struct shm_signal *s);
+	void     (*fault)(struct shm_signal *s, const char *fmt, ...);
+	void     (*release)(struct shm_signal *s);
+};
+
+enum {
+	shm_signal_in_wakeup,
+};
+
+struct shm_signal {
+	struct kref                 kref;
+	spinlock_t                  lock;
+	enum shm_signal_locality    locale;
+	unsigned long               flags;
+	struct shm_signal_ops      *ops;
+	struct shm_signal_desc     *desc;
+	struct shm_signal_notifier *notifier;
+	struct tasklet_struct       deferred_notify;
+};
+
+#define SHM_SIGNAL_FAULT(s, fmt, args...)  \
+  ((s)->ops->fault ? (s)->ops->fault((s), fmt, ## args) : panic(fmt, ## args))
+
+ /*
+  * These functions should only be used internally
+  */
+void _shm_signal_release(struct kref *kref);
+void _shm_signal_wakeup(struct shm_signal *s);
+
+/**
+ * shm_signal_init() - initialize an SHM_SIGNAL
+ * @s:        SHM_SIGNAL context
+ *
+ * Initializes SHM_SIGNAL context before first use
+ *
+ **/
+void shm_signal_init(struct shm_signal *s, enum shm_signal_locality locale,
+		     struct shm_signal_ops *ops, struct shm_signal_desc *desc);
+
+/**
+ * shm_signal_get() - acquire an SHM_SIGNAL context reference
+ * @s:        SHM_SIGNAL context
+ *
+ **/
+static inline struct shm_signal *shm_signal_get(struct shm_signal *s)
+{
+	kref_get(&s->kref);
+
+	return s;
+}
+
+/**
+ * shm_signal_put() - release an SHM_SIGNAL context reference
+ * @s:        SHM_SIGNAL context
+ *
+ **/
+static inline void shm_signal_put(struct shm_signal *s)
+{
+	kref_put(&s->kref, _shm_signal_release);
+}
+
+/**
+ * shm_signal_enable() - enables local notifications on an SHM_SIGNAL
+ * @s:        SHM_SIGNAL context
+ * @flags:      Reserved for future use, must be 0
+ *
+ * Enables/unmasks the registered notifier (if applicable) to receive wakeups
+ * whenever the remote side performs an shm_signal() operation. A notification
+ * will be dispatched immediately if any pending signals have already been
+ * issued prior to invoking this call.
+ *
+ * This is synonymous with unmasking an interrupt.
+ *
+ * Returns: success = 0, <0 = ERRNO
+ *
+ **/
+int shm_signal_enable(struct shm_signal *s, int flags);
+
+/**
+ * shm_signal_disable() - disable local notifications on an SHM_SIGNAL
+ * @s:        SHM_SIGNAL context
+ * @flags:      Reserved for future use, must be 0
+ *
+ * Disables/masks the registered shm_signal_notifier (if applicable) from
+ * receiving any further notifications.  Any subsequent calls to shm_signal()
+ * by the remote side will update the shm as dirty, but will not traverse the
+ * locale boundary and will not invoke the notifier callback.  Signals
+ * delivered while masked will be deferred until shm_signal_enable() is
+ * invoked.
+ *
+ * This is synonymous with masking an interrupt
+ *
+ * Returns: success = 0, <0 = ERRNO
+ *
+ **/
+int shm_signal_disable(struct shm_signal *s, int flags);
+
+/**
+ * shm_signal_inject() - notify the remote side about shm changes
+ * @s:        SHM_SIGNAL context
+ * @flags:      Reserved for future use, must be 0
+ *
+ * Marks the shm state as "dirty" and, if enabled, will traverse
+ * a locale boundary to inject a remote notification.  The remote
+ * side controls whether the notification should be delivered via
+ * the shm_signal_enable/disable() interface.
+ *
+ * The specifics of how to traverse a locale boundary are abstracted
+ * by the shm_signal_ops->signal() interface and provided by a particular
+ * implementation.  However, typically going north to south would be
+ * something like a syscall/hypercall, and going south to north would be
+ * something like a posix-signal/guest-interrupt.
+ *
+ * Returns: success = 0, <0 = ERRNO
+ *
+ **/
+int shm_signal_inject(struct shm_signal *s, int flags);
+
+#endif /* __KERNEL__ */
+
+#endif /* _LINUX_SHM_SIGNAL_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index bb1326d..136da19 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -200,4 +200,13 @@ config NLATTR
 config GENERIC_ATOMIC64
        bool
 
+config SHM_SIGNAL
+	tristate "SHM Signal - Generic shared-memory signaling mechanism"
+	default n
+	help
+	 Provides a shared-memory based signaling mechansim to indicate
+         memory-dirty notifications between two end-points.
+
+	 If unsure, say N
+
 endmenu
diff --git a/lib/Makefile b/lib/Makefile
index 2e78277..503bf7b 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -76,6 +76,7 @@ obj-$(CONFIG_TEXTSEARCH_BM) += ts_bm.o
 obj-$(CONFIG_TEXTSEARCH_FSM) += ts_fsm.o
 obj-$(CONFIG_SMP) += percpu_counter.o
 obj-$(CONFIG_AUDIT_GENERIC) += audit.o
+obj-$(CONFIG_SHM_SIGNAL) += shm_signal.o
 
 obj-$(CONFIG_SWIOTLB) += swiotlb.o
 obj-$(CONFIG_IOMMU_HELPER) += iommu-helper.o
diff --git a/lib/shm_signal.c b/lib/shm_signal.c
new file mode 100644
index 0000000..fbba74f
--- /dev/null
+++ b/lib/shm_signal.c
@@ -0,0 +1,192 @@
+/*
+ * Copyright 2009 Novell.  All Rights Reserved.
+ *
+ * See include/linux/shm_signal.h for documentation
+ *
+ * Author:
+ *      Gregory Haskins <[email-masked]>
+ *
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License
+ * as published by the Free Software Foundation.
+ *
+ * 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 St, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#include <linux/module.h>
+#include <linux/interrupt.h>
+#include <linux/shm_signal.h>
+
+int shm_signal_enable(struct shm_signal *s, int flags)
+{
+	struct shm_signal_irq *irq = &s->desc->irq[s->locale];
+	unsigned long iflags;
+
+	spin_lock_irqsave(&s->lock, iflags);
+
+	irq->enabled = 1;
+	wmb();
+
+	if ((irq->dirty || irq->pending)
+	    && !test_bit(shm_signal_in_wakeup, &s->flags)) {
+		rmb();
+		tasklet_schedule(&s->deferred_notify);
+	}
+
+	spin_unlock_irqrestore(&s->lock, iflags);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(shm_signal_enable);
+
+int shm_signal_disable(struct shm_signal *s, int flags)
+{
+	struct shm_signal_irq *irq = &s->desc->irq[s->locale];
+
+	irq->enabled = 0;
+	wmb();
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(shm_signal_disable);
+
+/*
+ * signaling protocol:
+ *
+ * each side of the shm_signal has an "irq" structure with the following
+ * fields:
+ *
+ *    - enabled: controlled by shm_signal_enable/disable() to mask/unmask
+ *               the notification locally
+ *    - dirty:   indicates if the shared-memory is dirty or clean.  This
+ *               is updated regardless of the enabled/pending state so that
+ *               the state is always accurately tracked.
+ *    - pending: indicates if a signal is pending to the remote locale.
+ *               This allows us to determine if a remote-notification is
+ *               already in flight to optimize spurious notifications away.
+ */
+int shm_signal_inject(struct shm_signal *s, int flags)
+{
+	/* Load the irq structure from the other locale */
+	struct shm_signal_irq *irq = &s->desc->irq[!s->locale];
+
+	/*
+	 * We always mark the remote side as dirty regardless of whether
+	 * they need to be notified.
+	 */
+	irq->dirty = 1;
+	wmb();   /* dirty must be visible before we test the pending state */
+
+	if (irq->enabled && !irq->pending) {
+		rmb();
+
+		/*
+		 * If the remote side has enabled notifications, and we do
+		 * not see a notification pending, we must inject a new one.
+		 */
+		irq->pending = 1;
+		wmb(); /* make it visible before we do the injection */
+
+		s->ops->inject(s);
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(shm_signal_inject);
+
+void _shm_signal_wakeup(struct shm_signal *s)
+{
+	struct shm_signal_irq *irq = &s->desc->irq[s->locale];
+	int dirty;
+	unsigned long flags;
+
+	spin_lock_irqsave(&s->lock, flags);
+
+	__set_bit(shm_signal_in_wakeup, &s->flags);
+
+	/*
+	 * The outer loop protects against race conditions between
+	 * irq->dirty and irq->pending updates
+	 */
+	while (irq->enabled && (irq->dirty || irq->pending)) {
+
+		/*
+		 * Run until we completely exhaust irq->dirty (it may
+		 * be re-dirtied by the remote side while we are in the
+		 * callback).  We let "pending" remain untouched until we have
+		 * processed them all so that the remote side knows we do not
+		 * need a new notification (yet).
+		 */
+		do {
+			irq->dirty = 0;
+			/* the unlock is an implicit wmb() for dirty = 0 */
+			spin_unlock_irqrestore(&s->lock, flags);
+
+			if (s->notifier)
+				s->notifier->signal(s->notifier);
+
+			spin_lock_irqsave(&s->lock, flags);
+			dirty = irq->dirty;
+			rmb();
+
+		} while (irq->enabled && dirty);
+
+		barrier();
+
+		/*
+		 * We can finally acknowledge the notification by clearing
+		 * "pending" after all of the dirty memory has been processed
+		 * Races against this clearing are handled by the outer loop.
+		 * Subsequent iterations of this loop will execute with
+		 * pending=0 potentially leading to future spurious
+		 * notifications, but this is an acceptable tradeoff as this
+		 * will be rare and harmless.
+		 */
+		irq->pending = 0;
+		wmb();
+
+	}
+
+	__clear_bit(shm_signal_in_wakeup, &s->flags);
+	spin_unlock_irqrestore(&s->lock, flags);
+
+}
+EXPORT_SYMBOL_GPL(_shm_signal_wakeup);
+
+void _shm_signal_release(struct kref *kref)
+{
+	struct shm_signal *s = container_of(kref, struct shm_signal, kref);
+
+	s->ops->release(s);
+}
+EXPORT_SYMBOL_GPL(_shm_signal_release);
+
+static void
+deferred_notify(unsigned long data)
+{
+	struct shm_signal *s = (struct shm_signal *)data;
+
+	_shm_signal_wakeup(s);
+}
+
+void shm_signal_init(struct shm_signal *s, enum shm_signal_locality locale,
+		     struct shm_signal_ops *ops, struct shm_signal_desc *desc)
+{
+	memset(s, 0, sizeof(*s));
+	kref_init(&s->kref);
+	spin_lock_init(&s->lock);
+	tasklet_init(&s->deferred_notify,
+		     deferred_notify,
+		     (unsigned long)s);
+	s->locale   = locale;
+	s->ops      = ops;
+	s->desc     = desc;
+}
+EXPORT_SYMBOL_GPL(shm_signal_init);