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Subject[patch] x86, mtrr: avoid MTRR reprogramming on BP during boot on UP platforms
Markus Kohn ran into a hard hang regression on an acer aspire 1310, when acpi is
enabled. git bisect showed the following commit as the bad one that introduced
the boot regression.

commit d0af9eed5aa91b6b7b5049cae69e5ea956fd85c3
Author: Suresh Siddha <>
Date: Wed Aug 19 18:05:36 2009 -0700

x86, pat/mtrr: Rendezvous all the cpus for MTRR/PAT init

Because of the UP configuration of that platform, native_smp_prepare_cpus()
bailed out (in smp_sanity_check()) before doing the set_mtrr_aps_delayed_init()

Further down the boot path, native_smp_cpus_done() will call the delayed
MTRR initialization for the AP's (mtrr_aps_init()) with mtrr_aps_delayed_init
not set. This resulted in the boot processor reprogramming its MTRR's to
the values seen during the start of the OS boot. While this is not needed
ideally, this shouldn't have caused any side-effects. This is because the
reprogramming of MTRR's (set_mtrr_state() that gets called via set_mtrr()) will
check if the live register contents are different from what is being
asked to write and will do the actual write only if they are different.

BP's mtrr state is read during the start of the OS boot and typically nothing
would have changed when we ask to reprogram it on BP again because of the above
scenario on an UP platform. So on a normal UP platform no reprogramming of BP
MTRR MSR's happens and all is well.

However, on this platform, bios seems to be modifying the fixed mtrr range
registers between the start of OS boot and when we double check the live
registers for reprogramming BP MTRR registers. And as the live registers are
modified, we end up reprogramming the MTRR's to the state seen during
the start of the OS boot.

During ACPI initialization, something in the bios (probably smi handler?) don't
like this fact and results in a hard lockup.

We didn't see this boot hang issue on this platform before the commit
d0af9eed5aa91b6b7b5049cae69e5ea956fd85c3, because only the AP's (if any)
will program its MTRR's to the value that BP had at the start of the OS boot.

Fix this issue by checking mtrr_aps_delayed_init before continuing
further in the mtrr_aps_init(). Now, only AP's (if any) will program
its MTRR's to the BP values during boot.


[By the way, this behavior of the bios modifying MTRR's after the start
of the OS boot is not common and the kernel is not prepared to
handle this situation well. Irrespective of this issue, during
suspend/resume, linux kernel will try to reprogram the BP's MTRR values
to the values seen during the start of the OS boot. So suspend/resume might
be already broken on this platform for all linux kernel versions.]

Reported-and-bisected-by: Markus Kohn <>
Signed-off-by: Suresh Siddha <>
Cc: Thomas Renninger <>
Cc: Rafael Wysocki <>
Cc: Venkatesh Pallipadi <>
Cc: [v2.6.32+]
arch/x86/kernel/cpu/mtrr/main.c | 10 +++++++++-
1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
index 01c0f3e..4fe5ebc 100644
--- a/arch/x86/kernel/cpu/mtrr/main.c
+++ b/arch/x86/kernel/cpu/mtrr/main.c
@@ -793,13 +793,21 @@ void set_mtrr_aps_delayed_init(void)

- * MTRR initialization for all AP's
+ * Delayed MTRR initialization for all AP's
void mtrr_aps_init(void)
if (!use_intel())

+ /*
+ * Check if someone has requested the delay of AP MTRR initialization,
+ * by doing set_mtrr_aps_delayed_init(), prior to this point. If not,
+ * then we are done.
+ */
+ if (!mtrr_aps_delayed_init)
+ return;
set_mtrr(~0U, 0, 0, 0);
mtrr_aps_delayed_init = false;

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