[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/sanboot.c

@@ -41,6 +41,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/iso9660.h>
 #include <ipxe/dhcp.h>
 #include <ipxe/settings.h>
+#include <ipxe/quiesce.h>
 #include <ipxe/sanboot.h>
 
 /**
@@ -365,6 +366,9 @@ int sandev_reopen ( struct san_device *sandev ) {
 	struct san_path *sanpath;
 	int rc;
 
+	/* Unquiesce system */
+	unquiesce();
+
 	/* Close any outstanding command and restart interfaces */
 	sandev_restart ( sandev, -ECONNRESET );
 	assert ( sandev->active == NULL );
@@ -503,6 +507,9 @@ sandev_command ( struct san_device *sandev,
 	/* Sanity check */
 	assert ( ! timer_running ( &sandev->timer ) );
 
+	/* Unquiesce system */
+	unquiesce();
+
 	/* (Re)try command */
 	do {
 
@@ -654,6 +661,14 @@ int sandev_write ( struct san_device *sandev, uint64_t lba,
 	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;
 }