path: root/arch/ia64
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authorTony Luck <tony.luck@intel.com>2014-03-28 14:42:07 -0700
committerTony Luck <tony.luck@intel.com>2014-04-16 10:20:34 -0700
commitc0b5a64d937f8fff79b10410a8a70ecaa9a1c580 (patch)
tree88ab6443b1a2226a8d488ee63d15b03f6b650a9d /arch/ia64
parentb098d6726bbfb94c06d6e1097466187afddae61f (diff)
[IA64] Change default PSR.ac from '1' to '0' (Fix erratum #237)
April 2014 Itanium processor specification update: http://www.intel.com/content/www/us/en/processors/itanium/itanium-specification-update.html describes this erratum: ========================================================================= 237. Under a complex set of conditions, store to load forwarding for a sub 8-byte load may complete incorrectly Problem: A load instruction may complete incorrectly when a code sequence using 4-byte or smaller load and store operations to the same address is executed in combination with specific timing of all the following concurrent conditions: store to load forwarding, alignment checking enabled, a mis-predicted branch, and complex cache utilization activity. Implication: The affected sub 8-byte instruction may complete incorrectly resulting in unpredictable system behavior. There is an extremely low probability of exposure due to the significant number of complex microarchitectural concurrent conditions required to encounter the erratum. Workaround: Set PSR.ac = 0 to completely avoid the erratum. Disabling Hyper-Threading will significantly reduce exposure to the conditions that contribute to encountering the erratum. Status: See the Summary Table of Changes for the affected steppings. ========================================================================= [Table of changes essentially lists all models from McKinley to Tukwila] The PSR.ac bit controls whether the processor will always generate an unaligned reference trap (0x5a00) for a misaligned data access (when PSR.ac=1) or if it will let the access succeed when running on a cpu that implements logic to handle some unaligned accesses. Way back in 2008 in commit b704882e70d87d7f56db5ff17e2253f3fa90e4f3 [IA64] Rationalize kernel mode alignment checking we made the decision to always enable strict checking. We were already doing so in trap/interrupt context because the common preamble code set this bit - but the rest of supervisor code (and by inheritance user code) ran with PSR.ac=0. We now reverse that decision and set PSR.ac=0 everywhere in the kernel (also inherited by user processes). This will avoid the erratum using the method described in the Itanium specification update. Net effect for users is that the processor will handle unaligned access when it can (typically with a tiny performance bubble in the pipeline ... but much less invasive than taking a trap and having the OS perform the access). Signed-off-by: Tony Luck <tony.luck@intel.com>
Diffstat (limited to 'arch/ia64')
3 files changed, 3 insertions, 3 deletions
diff --git a/arch/ia64/kernel/head.S b/arch/ia64/kernel/head.S
index e6f80fcf013b..a4acddad0c78 100644
--- a/arch/ia64/kernel/head.S
+++ b/arch/ia64/kernel/head.S
@@ -259,7 +259,7 @@ start_ap:
* Switch into virtual mode:
+ |IA64_PSR_DI)
mov cr.ipsr=r16
movl r17=1f
diff --git a/arch/ia64/kernel/ivt.S b/arch/ia64/kernel/ivt.S
index 689ffcaa284e..18e794a57248 100644
--- a/arch/ia64/kernel/ivt.S
+++ b/arch/ia64/kernel/ivt.S
@@ -58,7 +58,7 @@
#include <asm/unistd.h>
#include <asm/errno.h>
-#if 1
+#if 0
# define PSR_DEFAULT_BITS psr.ac
diff --git a/arch/ia64/kvm/vmm_ivt.S b/arch/ia64/kvm/vmm_ivt.S
index 24018484c6e9..397e34a63e18 100644
--- a/arch/ia64/kvm/vmm_ivt.S
+++ b/arch/ia64/kvm/vmm_ivt.S
@@ -64,7 +64,7 @@
#include "kvm_minstate.h"
#include "vti.h"
-#if 1
+#if 0
# define PSR_DEFAULT_BITS psr.ac