[efi] Avoid dropping below TPL as at entry to iPXE

iPXE will currently drop to TPL_APPLICATION whenever the current
system time is obtained via currticks(), since the system time
mechanism relies on a timer that can fire only when the TPL is below
TPL_CALLBACK.

This can cause unexpected behaviour if the system time is obtained in
the middle of an API call into iPXE by external code.  For example,
MnpDxe sets up a 10ms periodic timer running at TPL_CALLBACK to poll
the underling EFI_SIMPLE_NETWORK_PROTOCOL device for received packets.
If the resulting poll within iPXE happens to hit a code path that
requires obtaining the current system time (e.g. due to reception of
an STP packet, which affects iPXE's blocked link timer), then iPXE
will end up temporarily dropping to TPL_APPLICATION.  This can
potentially result in retriggering the MnpDxe periodic timer, causing
code to be unexpectedly re-entered.

Fix by recording the external TPL at any entry point into iPXE and
dropping only as far as this external TPL, rather than dropping
unconditionally to TPL_APPLICATION.

The side effect of this change is that iPXE's view of the current
system time will be frozen for the duration of any API calls made into
iPXE by external code at TPL_CALLBACK or above.  Since any such
external code is already responsible for allowing execution at
TPL_APPLICATION to occur, then this should not cause a problem in
practice.

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

1 files changed, 12 insertions, 3 deletions

diff --git a/src/interface/efi/efi_timer.c b/src/interface/efi/efi_timer.c
index 8f40cb8..405cd34 100644
--- a/src/interface/efi/efi_timer.c
+++ b/src/interface/efi/efi_timer.c
@@ -97,8 +97,17 @@ static unsigned long efi_currticks ( void ) {
 	 * gain us any substantive benefits (since even with such
 	 * calls we would still be suffering from the limitations of a
 	 * polling design), we instead choose to run at TPL_CALLBACK
-	 * almost all of the time, dropping to TPL_APPLICATION to
-	 * allow timer ticks to occur.
+	 * almost all of the time, dropping to a lower TPL to allow
+	 * timer ticks to occur.
+	 *
+	 * We record the external TPL at the point of entry into iPXE,
+	 * and drop back only as far as this external TPL.  This
+	 * avoids the unexpected behaviour that may arise from having
+	 * iPXE temporarily drop to TPL_APPLICATION in the middle of
+	 * an entry point invoked at TPL_CALLBACK.  The side effect is
+	 * that iPXE's view of the system time is effectively frozen
+	 * for the duration of any call made in to iPXE at
+	 * TPL_CALLBACK or higher.
 	 *
 	 *
 	 * For added excitement, UEFI provides no clean way for device
@@ -127,7 +136,7 @@ static unsigned long efi_currticks ( void ) {
 	if ( efi_shutdown_in_progress ) {
 		efi_jiffies++;
 	} else {
-		bs->RestoreTPL ( TPL_APPLICATION );
+		bs->RestoreTPL ( efi_external_tpl );
 		bs->RaiseTPL ( TPL_CALLBACK );
 	}