x86/bus_lock: Don't assume the init value of DEBUGCTLMSR.BUS_LOCK_DETECT to be zero

It's possible that this kernel has been kexec'd from a kernel that
enabled bus lock detection, or (hypothetically) BIOS/firmware has set
DEBUGCTLMSR_BUS_LOCK_DETECT.

Disable bus lock detection explicitly if not wanted.

Fixes: ebb1064e ("x86/traps: Handle #DB for bus lock")
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20220802033206.21333-1-chenyi.qiang@intel.com

arch/x86/kernel/cpu/intel.c

@@ -1216,22 +1216,23 @@ static void bus_lock_init(void)
 {
 	u64 val;
 
-	/*
-	 * Warn and fatal are handled by #AC for split lock if #AC for
-	 * split lock is supported.
-	 */
-	if (!boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT) ||
-	    (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) &&
-	    (sld_state == sld_warn || sld_state == sld_fatal)) ||
-	    sld_state == sld_off)
+	if (!boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT))
 		return;
 
-	/*
-	 * Enable #DB for bus lock. All bus locks are handled in #DB except
-	 * split locks are handled in #AC in the fatal case.
-	 */
 	rdmsrl(MSR_IA32_DEBUGCTLMSR, val);
-	val |= DEBUGCTLMSR_BUS_LOCK_DETECT;
+
+	if ((boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) &&
+	    (sld_state == sld_warn || sld_state == sld_fatal)) ||
+	    sld_state == sld_off) {
+		/*
+		 * Warn and fatal are handled by #AC for split lock if #AC for
+		 * split lock is supported.
+		 */
+		val &= ~DEBUGCTLMSR_BUS_LOCK_DETECT;
+	} else {
+		val |= DEBUGCTLMSR_BUS_LOCK_DETECT;
+	}
+
 	wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
 }