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Subject[PATCH v5 09/18] 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, user
application developer needs to fix the split lock issue.

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

Kernel developer should check the warning, which contains helpful faulting
address, context, and callstack info, and fix the split lock issue
one by one. Then further split lock may be captured and fixed.

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

Signed-off-by: Fenghua Yu <>
arch/x86/kernel/traps.c | 31 +++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d26f9e9c3d83..69b6233e783e 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>
@@ -293,7 +294,37 @@ 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)
+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) !=
+ cond_local_irq_enable(regs);
+ if (!user_mode(regs)) {
+ /*
+ * Only split lock can generate #AC from kernel. Warn
+ * and disable #AC for split lock on current CPU.
+ */
+ msr_clear_bit(MSR_TEST_CTL,
+ WARN_ONCE(1, "A split lock issue is detected.\n");
+ return;
+ }
+ /* Handle #AC generated from user code. */
+ do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
+ error_code, BUS_ADRALN, NULL);
+ }
#undef IP

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