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[efi] Run at TPL_CALLBACK to protect against UEFI timers

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As noted in the comments, UEFI manages to combines the all of the
worst aspects of both a polling design (inefficiency and inability to
sleep until something interesting happens) and of an interrupt-driven
design (the complexity of code that could be preempted at any time,
thanks to UEFI timers).

This causes problems in particular for UEFI USB keyboards: the
keyboard driver calls UsbAsyncInterruptTransfer() to set up a periodic
timer which is used to poll the USB bus.  This poll may interrupt a
critical section within iPXE, typically resulting in list corruption
and either a hang or reboot.

Work around this problem by mirroring the BIOS design, in which we run
with interrupts disabled almost all of the time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2018-02-20 10:56:31 +00:00
parent 8dbb73a779
commit c89a446cf0
3 changed files with 49 additions and 52 deletions
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48
src/interface/efi/efi_usb.c
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@@ -64,50 +64,6 @@ static const char * efi_usb_direction_name ( EFI_USB_DATA_DIRECTION direction ){
******************************************************************************
*/
/**
* Poll USB bus
*
* @v usbdev EFI USB device
*/
static void efi_usb_poll ( struct efi_usb_device *usbdev ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
struct usb_bus *bus = usbdev->usb->port->hub->bus;
EFI_TPL tpl;
/* UEFI manages to ingeniously combine the worst aspects of
* both polling and interrupt-driven designs. There is no way
* to support proper interrupt-driven operation, since there
* is no way to hook in an interrupt service routine. A
* mockery of interrupts is provided by UEFI timers, which
* trigger at a preset rate and can fire at any time.
*
* We therefore have all of the downsides of a polling design
* (inefficiency and inability to sleep until something
* interesting happens) combined with all of the downsides of
* an interrupt-driven design (the complexity of code that
* could be preempted at any time).
*
* The UEFI specification expects us to litter the entire
* codebase with calls to RaiseTPL() as needed for sections of
* code that are not reentrant. Since this doesn't actually
* 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 wrap only calls to
* usb_poll(). This should be sufficient for most practical
* purposes.
*
* A "proper" solution would involve rearchitecting the whole
* codebase to support interrupt-driven operation.
*/
tpl = bs->RaiseTPL ( TPL_NOTIFY );
/* Poll bus */
usb_poll ( bus );
/* Restore task priority level */
bs->RestoreTPL ( tpl );
}
/**
* Poll USB bus (from endpoint event timer)
*
@@ -216,7 +172,7 @@ static int efi_usb_open ( struct efi_usb_interface *usbintf,
/* Create event */
if ( ( efirc = bs->CreateEvent ( ( EVT_TIMER | EVT_NOTIFY_SIGNAL ),
TPL_NOTIFY, efi_usb_timer, usbep,
TPL_CALLBACK, efi_usb_timer, usbep,
&usbep->event ) ) != 0 ) {
rc = -EEFI ( efirc );
DBGC ( usbdev, "USBDEV %s %s could not create event: %s\n",
@@ -363,7 +319,7 @@ static int efi_usb_sync_transfer ( struct efi_usb_interface *usbintf,
for ( i = 0 ; ( ( timeout == 0 ) || ( i < timeout ) ) ; i++ ) {
/* Poll bus */
efi_usb_poll ( usbdev );
usb_poll ( usbdev->usb->port->hub->bus );
/* Check for completion */
if ( usbep->rc != -EINPROGRESS ) {