Re: [PATCH v5 09/18] x86/split_lock: Handle #AC exception for split lock

[Fenghua Yu]

On Tue, Mar 12, 2019 at 04:51:22PM -0700, Dave Hansen wrote:
> On 3/12/19 4:00 PM, Fenghua Yu wrote:
> I don't see any feature checking here.  Don't we need to see if this MSR
> is supported?
> 
> Shouldn't the code here on systems that don't support split lock
> disabling be the same as on CONFIG_CPU_SUP_INTEL=n systems?

You are right. Is the following #AC handler code better?

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d26f9e9c3d83..5296021509c7 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) !=
+			NOTIFY_STOP) {
+		cond_local_irq_enable(regs);
+		if (!user_mode(regs)) {
+			if (!this_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+				return;
+
+			/*
+			 * Only split lock can generate #AC from kernel. Warn
+			 * and disable #AC for split lock on current CPU.
+			 */
+			msr_clear_bit(MSR_TEST_CTL,
+				      TEST_CTL_ENABLE_SPLIT_LOCK_DETECT_SHIFT);
+			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
 
 #ifdef CONFIG_VMAP_STACK