[efi] Disable EFI watchdog timer when shutting down to boot an OS

The UEFI specification mandates that the EFI watchdog timer should be
disabled by the platform firmware as part of the ExitBootServices()
call, but some platforms (e.g. Hyper-V) are observed to occasionally
forget to do so, resulting in a reboot approximately five minutes
after starting the operating system.

Work around these firmware bugs by disabling the watchdog timer
ourselves.

Requested-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_watchdog.c

@@ -34,6 +34,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <string.h>
 #include <ipxe/retry.h>
 #include <ipxe/timer.h>
+#include <ipxe/init.h>
 #include <ipxe/efi/efi.h>
 #include <ipxe/efi/efi_watchdog.h>
 
@@ -80,3 +81,36 @@ static void efi_watchdog_expired ( struct retry_timer *timer,
 
 /** Watchdog holdoff timer */
 struct retry_timer efi_watchdog = TIMER_INIT ( efi_watchdog_expired );
+
+/**
+ * Disable watching when shutting down to boot an operating system
+ *
+ * @v booting		System is shutting down for OS boot
+ */
+static void efi_watchdog_shutdown ( int booting ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+	EFI_STATUS efirc;
+	int rc;
+
+	/* If we are shutting down to boot an operating system, then
+	 * disable the boot services watchdog timer.  The UEFI
+	 * specification mandates that the platform firmware does this
+	 * as part of the ExitBootServices() call, but some platforms
+	 * (e.g. Hyper-V) are observed to occasionally forget to do
+	 * so, resulting in a reboot approximately five minutes after
+	 * starting the operating system.
+	 */
+	if ( booting &&
+	     ( ( efirc = bs->SetWatchdogTimer ( 0, 0, 0, NULL ) ) != 0 ) ) {
+		rc = -EEFI ( efirc );
+		DBGC ( &efi_watchdog, "EFI could not disable watchdog timer: "
+		       "%s\n", strerror ( rc ) );
+		/* Nothing we can do */
+	}
+}
+
+/** Watchdog startup/shutdown function */
+struct startup_fn efi_watchdog_startup_fn __startup_fn ( STARTUP_EARLY ) = {
+	.name = "efi_watchdog",
+	.shutdown = efi_watchdog_shutdown,
+};