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Subject[PATCH v7 11/21] x86/split_lock: Handle #AC exception for split lock
There may be different considerations on how to handle #AC for split lock,
e.g. how to handle system hang caused by split lock issue in firmware,
how to emulate faulting instruction, etc. We use a simple method to
handle user and kernel split lock and may extend the method in the future.

When #AC exception for split lock is triggered from user process, the
process is killed by SIGBUS. To execute the process properly, a user
application developer needs to fix the split lock issue.

When #AC exception for split lock is triggered from a kernel instruction,
disable split lock detection on local CPU and warn the split lock issue.
After the exception, the faulting instruction will be executed and kernel
execution continues. Split lock detection is only disabled on the local
CPU, not globally. It will be re-enabled if the CPU is offline and then
online or through sysfs interface.

A kernel/driver developer should check the warning, which contains helpful
faulting address, context, and callstack info, and fix the split lock
issues. Then further split lock issues may be captured and fixed.

After bit 29 in MSR_TEST_CTL is set to 1 in kernel, firmware inherits
the setting when firmware is executed in S4, S5, run time services, SMI,
etc. If there is a split lock operation in firmware, it will triggers
#AC and may hang the system depending on how firmware handles the #AC.
It's up to a firmware developer to fix split lock issues in firmware.

Signed-off-by: Fenghua Yu <>
arch/x86/kernel/traps.c | 35 ++++++++++++++++++++++++++++++++++-
1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d26f9e9c3d83..c776bc0a47f5 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -61,6 +61,7 @@
#include <asm/mpx.h>
#include <asm/vm86.h>
#include <asm/umip.h>
+#include <asm/cpu.h>

#ifdef CONFIG_X86_64
#include <asm/x86_init.h>
@@ -293,9 +294,41 @@ DO_ERROR(X86_TRAP_OLD_MF, SIGFPE, 0, NULL, "coprocessor segment overru
DO_ERROR(X86_TRAP_TS, SIGSEGV, 0, NULL, "invalid TSS", invalid_TSS)
DO_ERROR(X86_TRAP_NP, SIGBUS, 0, NULL, "segment not present", segment_not_present)
DO_ERROR(X86_TRAP_SS, SIGBUS, 0, NULL, "stack segment", stack_segment)
-DO_ERROR(X86_TRAP_AC, SIGBUS, BUS_ADRALN, NULL, "alignment check", alignment_check)
#undef IP

+dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
+ unsigned int trapnr = X86_TRAP_AC;
+ char str[] = "alignment check";
+ int signr = SIGBUS;
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+ if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) ==
+ return;
+ cond_local_irq_enable(regs);
+ if (!user_mode(regs) &&
+ static_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT)) {
+ /*
+ * Only split lock can generate #AC from kernel at this point.
+ * Warn and disable split lock detection on this CPU. The
+ * faulting instruction will be executed without generating
+ * another #AC fault.
+ */
+ this_cpu_and(msr_test_ctl_cache, ~TEST_CTL_SPLIT_LOCK_DETECT);
+ WARN_ONCE(1, "split lock operation detected\n");
+ return;
+ }
+ /* Handle #AC generated in any other cases. */
+ do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
+ error_code, BUS_ADRALN, NULL);
__visible void __noreturn handle_stack_overflow(const char *message,
struct pt_regs *regs,
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