[SRU][Bionic/Focal][PATCH 0/2] limit "Dummy wait" to old Intel systems

[Jeff Lane]

BugLink: https://bugs.launchpad.net/bugs/1990985

Old, circa 2002 chipsets have a bug: they don't go idle when they are
supposed to. So, a workaround was added to slow the CPU down and
ensure that the CPU waits a bit for the chipset to actually go idle.
This workaround is ancient and has been in place in some form since
the original kernel ACPI implementation.

But, this workaround is very painful on modern systems. The "inl()"
can take thousands of cycles (see Link: for some more detailed
numbers and some fun kernel archaeology).

First and foremost, modern systems should not be using this code.
Typical Intel systems have not used it in over a decade because it is
horribly inferior to MWAIT-based idle.

Despite this, people do seem to be tripping over this workaround on
AMD system today.

Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD
systems from tripping over the workaround. Remotely modern Intel
systems use intel_idle instead of this code and will, in practice,
remain unaffected by the dummy wait.

Small patch cleanly picks to both Bionic and Focal.



Dave Hansen (1):
  ACPI: processor idle: Practically limit "Dummy wait" workaround to old
    Intel systems

Yin Fengwei (1):
  ACPI: processor_idle: Skip dummy wait if kernel is in guest

 drivers/acpi/processor_idle.c | 38 +++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

-- 
2.34.1

old/0000-cover-letter.patch
>From 002fccdc72f62dd7c365f6f44ccb6f8759b8ab38 Mon Sep 17 00:00:00 2001
From: Jeff Lane <jeffrey.lane at canonical.com>
Date: Mon, 3 Oct 2022 19:09:20 -0400
 

Dave Hansen (1):
  ACPI: processor idle: Practically limit "Dummy wait" workaround to old
    Intel systems

Yin Fengwei (1):
  ACPI: processor_idle: Skip dummy wait if kernel is in guest

 drivers/acpi/processor_idle.c | 38 +++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

-- 
2.34.1