[block] Provide abstraction to allow system to be quiesced

When performing a SAN boot via INT 13, there is no way for the
operating system to indicate that it has finished using the INT 13 SAN
device.  We therefore have no opportunity to clean up state before the
loaded operating system's native drivers take over.  This can cause
problems when booting Windows, which tends not to be forgiving of
unexpected system state.

Windows will typically write a flag to the SAN device as the last
action before transferring control to the native drivers.  We can use
this as a heuristic to bring the system to a quiescent state (without
performing a full shutdown); this provides us an opportunity to
temporarily clean up state that could otherwise prevent a successful
Windows boot.

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

src/core/quiesce.c

+/*
+ * Copyright (C) 2017 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 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.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/**
+ * @file
+ *
+ * Quiesce system
+ *
+ */
+
+#include <ipxe/quiesce.h>
+
+/** Quiesce system */
+void quiesce ( void ) {
+	struct quiescer *quiescer;
+
+	/* Call all quiescers */
+	for_each_table_entry ( quiescer, QUIESCERS ) {
+		quiescer->quiesce();
+	}
+}
+
+/** Unquiesce system */
+void unquiesce ( void ) {
+	struct quiescer *quiescer;
+
+	/* Call all quiescers */
+	for_each_table_entry ( quiescer, QUIESCERS ) {
+		quiescer->unquiesce();
+	}
+}

src/core/sanboot.c

 #include <ipxe/iso9660.h>
 #include <ipxe/dhcp.h>
 #include <ipxe/settings.h>
+#include <ipxe/quiesce.h>
 #include <ipxe/sanboot.h>
 
 /**
 	struct san_path *sanpath;
 	int rc;
 
+	/* Unquiesce system */
+	unquiesce();
+
 	/* Close any outstanding command and restart interfaces */
 	sandev_restart ( sandev, -ECONNRESET );
 	assert ( sandev->active == NULL );
 	/* Sanity check */
 	assert ( ! timer_running ( &sandev->timer ) );
 
+	/* Unquiesce system */
+	unquiesce();
+
 	/* (Re)try command */
 	do {
 
 	if ( ( rc = sandev_rw ( sandev, lba, count, buffer, block_write ) ) != 0 )
 		return rc;
 
+	/* Quiesce system.  This is a heuristic designed to ensure
+	 * that the system is quiesced before Windows starts up, since
+	 * a Windows SAN boot will typically write a status flag to
+	 * the disk as its last action before transferring control to
+	 * the native drivers.
+	 */
+	quiesce();
+
 	return 0;
 }
 

src/include/ipxe/quiesce.h

+#ifndef _IPXE_QUIESCE_H
+#define _IPXE_QUIESCE_H
+
+/** @file
+ *
+ * Quiesce system
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/tables.h>
+
+/** A quiescer */
+struct quiescer {
+	/** Quiesce system */
+	void ( * quiesce ) ( void );
+	/** Unquiesce system */
+	void ( * unquiesce ) ( void );
+};
+
+/** Quiescer table */
+#define QUIESCERS __table ( struct quiescer, "quiescers" )
+
+/** Declare a quiescer */
+#define __quiescer __table_entry ( QUIESCERS, 01 )
+
+extern void quiesce ( void );
+extern void unquiesce ( void );
+
+#endif /* _IPXE_QUIESCE_H */