Matthew Garrett (mjg59) wrote,

AE_AML_BUFFER_LIMIT in \_SB_._OSC

If you get messages like this:

ACPI Error (psparse-0537): Method parse/execution failed [\_SB_._OSC] (Node ffff8801e8c62b30), AE_AML_BUFFER_LIMIT

then you've fallen foul of one of the less appealing aspects of ACPI. _OSC methods are defined as methods to allow the operating system and the firmware to handshake over their support of optional features. Different _OSC methods apply to different types of hardware. CPU _OSC methods allow the operating system to inform the firmware that it supports ACPI 3.0 throttling states, while PCIe _OSC methods allow the operating system to indicate that it can manage native PCIe hotplug. The firmware can choose whether or not to give up control of these features, and the OS then has to cope.

The problem arises when we get to the _OSC method on the system bus. This wasn't specified until ACPI 4.0, leaving an attractive mechanism for vendors to add OS/firmware integration. To that end, we now have at least three different _SB_._OSC methods in the wild:
  1. The ACPI specified _OSC. This exists purely for the OS to tell the firmware what it supports, without the firmware having the opportunity to disagree. As such it passes 8 bytes of data to the method.
  2. The Microsoft WHEA _OSC. This exists to allow the firmware and Windows to handshake over whether or not the firmware supports Microsoft's hardware error reporting.
  3. HP's PCC _OSC method, designed to allow handshaking between the OS and the firmware in order to determine whether they support OS interaction with the firmware-level CPU scaling


This wouldn't be enough to be a problem in itself. The ACPI spec requires _OSC methods to have GUIDs in order to protect users from exactly this kind of situation - Windows can attempt to enable WHEA on a machine with a spec-compliant _OSC, and the _OSC method will return an immediate failure because the GUID doesn't match. Except that the WHEA and PCC versions of _SB_._OSC pass 12 bytes of data in the third argument, against the ACPI spec version's 8. And many _OSC implementations attempt to access the region between bytes 9 and 12 before checking the GUID, resulting in the AE_AML_BUFFER_LIMIT error.

This is made even more annoying due to the fact that argument 2 contains the number of parameters being passed in argument 3, making it straightforward to avoid this kind of failure. Firmware authors, tbh.
Tags: advogato, fedora
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