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SubjectRe: CPU MTRRs and linux kernel
On Tue, Jun 12, 2012 at 02:37:40AM -0700, Mahmood Naderan wrote:
> Hi everybody,
> Abour rhis warnning in boot log:
> WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 61952MB of RAM
> I
> searched a lot however didn't find a clear solution. Does AMD new
> processors (62XX) have problems with new kernels or old kernels??
> Mine is:
> Linux n1 2.6.32-24-server #39-Ubuntu SMP Wed Jul 28 06:21:40 UTC 2010 x86_64 GNU/Linux
> Should I upgrade?

Yes, fix is below and it got backported to -stable AFAICT but obviously
the ubuntus don't have it in the kernel you cite above.

commit 3fdbf004c1706480a7c7fac3c9d836fa6df20d7d
Author: Andreas Herrmann <>
Date: Thu Sep 30 14:32:35 2010 +0200

x86, mtrr: Assume SYS_CFG[Tom2ForceMemTypeWB] exists on all future AMD CPUs

Instead of adapting the CPU family check in amd_special_default_mtrr()
for each new CPU family assume that all new AMD CPUs support the
necessary bits in SYS_CFG MSR.

Tom2Enabled is architectural (defined in APM Vol.2).
Tom2ForceMemTypeWB is defined in all BKDGs starting with K8 NPT.
In pre K8-NPT BKDG this bit is reserved (read as zero).

W/o this adaption Linux would unnecessarily complain about bad MTRR
settings on every new AMD CPU family, e.g.

[ 0.000000] WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 4863MB of RAM.
Cc: # .32.x, .35.x
Signed-off-by: Andreas Herrmann <>
LKML-Reference: <>
Signed-off-by: H. Peter Anvin <>

diff --git a/arch/x86/kernel/cpu/mtrr/cleanup.c b/arch/x86/kernel/cpu/mtrr/cleanup.c
index c5f59d071425..ac140c7be396 100644
--- a/arch/x86/kernel/cpu/mtrr/cleanup.c
+++ b/arch/x86/kernel/cpu/mtrr/cleanup.c
@@ -827,7 +827,7 @@ int __init amd_special_default_mtrr(void)

if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
return 0;
- if (boot_cpu_data.x86 < 0xf || boot_cpu_data.x86 > 0x11)
+ if (boot_cpu_data.x86 < 0xf)
return 0;
/* In case some hypervisor doesn't pass SYSCFG through: */
if (rdmsr_safe(MSR_K8_SYSCFG, &l, &h) < 0)
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