[efi] Run ExitBootServices shutdown hook at TPL_NOTIFY

On some systems (observed with the Thunderbolt ports on a ThinkPad X1
Extreme Gen3 and a ThinkPad P53), if the IOMMU is enabled then the
system firmware will install an ExitBootServices notification event
that disables bus mastering on the Thunderbolt xHCI controller and all
PCI bridges, and destroys any extant IOMMU mappings.  This leaves the
xHCI controller unable to perform any DMA operations.

As described in commit 236299b ("[xhci] Avoid DMA during shutdown if
firmware has disabled bus mastering"), any subsequent DMA operation
attempted by the xHCI controller will end up completing after the
operating system kernel has reenabled bus mastering, resulting in a
DMA operation to an area of memory that the hardware is no longer
permitted to access and, on Windows with the Driver Verifier enabled,
a STOP 0xE6 (DRIVER_VERIFIER_DMA_VIOLATION).

That commit avoids triggering any DMA attempts during the shutdown of
the xHCI controller itself.  However, this is not a complete solution
since any attached and opened USB device (e.g. a USB NIC) may
asynchronously trigger DMA attempts that happen to occur after bus
mastering has been disabled but before we reset the xHCI controller.

Avoid this problem by installing our own ExitBootServices notification
event at TPL_NOTIFY, thereby causing it to be invoked before the
firmware's own ExitBootServices notification event that disables bus
mastering.

This unsurprisingly causes the shutdown hook itself to be invoked at
TPL_NOTIFY, which causes a fatal error when later code attempts to
raise the TPL to TPL_CALLBACK (which is a lower TPL).  Work around
this problem by redefining the "internal" iPXE TPL to be variable, and
set this internal TPL to TPL_NOTIFY when the shutdown hook is invoked.

Avoid calling into an underlying SNP protocol instance from within our
shutdown hook at TPL_NOTIFY, since the underlying SNP driver may
attempt to raise the TPL to TPL_CALLBACK (which would cause a fatal
error).  Failing to shut down the underlying SNP device is safe to do
since the underlying device must, in any case, have installed its own
ExitBootServices hook if any shutdown actions are required.

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

1 files changed, 1 insertions, 1 deletions

diff --git a/src/drivers/net/efi/nii.c b/src/drivers/net/efi/nii.c
index b9f34650..833462e7 100644
--- a/src/drivers/net/efi/nii.c
+++ b/src/drivers/net/efi/nii.c
@@ -576,7 +576,7 @@ static int nii_issue_cpb_db ( struct nii_nic *nii, unsigned int op, void *cpb,
 	cdb.IFnum = nii->nii->IfNum;
 
 	/* Raise task priority level */
-	tpl = bs->RaiseTPL ( TPL_CALLBACK );
+	tpl = bs->RaiseTPL ( efi_internal_tpl );
 
 	/* Issue command */
 	DBGC2 ( nii, "NII %s issuing %02x:%04x ifnum %d%s%s\n",