Merge tag 'x86-splitlock-2021-04-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 bus lock detection updates from Thomas Gleixner:
 "Support for enhanced split lock detection:

  Newer CPUs provide a second mechanism to detect operations with lock
  prefix which go accross a cache line boundary. Such operations have to
  take bus lock which causes a system wide performance degradation when
  these operations happen frequently.

  The new mechanism is not using the #AC exception. It triggers #DB and
  is restricted to operations in user space. Kernel side split lock
  access can only be detected by the #AC based variant.

  Contrary to the #AC based mechanism the #DB based variant triggers
  _after_ the instruction was executed. The mechanism is CPUID
  enumerated and contrary to the #AC version which is based on the magic
  TEST_CTRL_MSR and model/family based enumeration on the way to become
  architectural"

* tag 'x86-splitlock-2021-04-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  Documentation/admin-guide: Change doc for split_lock_detect parameter
  x86/traps: Handle #DB for bus lock
  x86/cpufeatures: Enumerate #DB for bus lock detection

3 files changed, 97 insertions, 20 deletions

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 340107800b36..99e1656b326e 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1330,7 +1330,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	cpu_set_bug_bits(c);
 
-	cpu_set_core_cap_bits(c);
+	sld_setup(c);
 
 	fpu__init_system(c);
 
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 63e381a46153..fe0bec14d7ec 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -44,9 +44,9 @@ enum split_lock_detect_state {
 };
 
 /*
- * Default to sld_off because most systems do not support split lock detection
- * split_lock_setup() will switch this to sld_warn on systems that support
- * split lock detect, unless there is a command line override.
+ * Default to sld_off because most systems do not support split lock detection.
+ * sld_state_setup() will switch this to sld_warn on systems that support
+ * split lock/bus lock detect, unless there is a command line override.
  */
 static enum split_lock_detect_state sld_state __ro_after_init = sld_off;
 static u64 msr_test_ctrl_cache __ro_after_init;
@@ -603,6 +603,7 @@ static void init_intel_misc_features(struct cpuinfo_x86 *c)
 }
 
 static void split_lock_init(void);
+static void bus_lock_init(void);
 
 static void init_intel(struct cpuinfo_x86 *c)
 {
@@ -720,6 +721,7 @@ static void init_intel(struct cpuinfo_x86 *c)
 		tsx_disable();
 
 	split_lock_init();
+	bus_lock_init();
 
 	intel_init_thermal(c);
 }
@@ -1020,16 +1022,15 @@ static bool split_lock_verify_msr(bool on)
 	return ctrl == tmp;
 }
 
-static void __init split_lock_setup(void)
+static void __init sld_state_setup(void)
 {
 	enum split_lock_detect_state state = sld_warn;
 	char arg[20];
 	int i, ret;
 
-	if (!split_lock_verify_msr(false)) {
-		pr_info("MSR access failed: Disabled\n");
+	if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) &&
+	    !boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT))
 		return;
-	}
 
 	ret = cmdline_find_option(boot_command_line, "split_lock_detect",
 				  arg, sizeof(arg));
@@ -1041,17 +1042,14 @@ static void __init split_lock_setup(void)
 			}
 		}
 	}
+	sld_state = state;
+}
 
-	switch (state) {
-	case sld_off:
-		pr_info("disabled\n");
+static void __init __split_lock_setup(void)
+{
+	if (!split_lock_verify_msr(false)) {
+		pr_info("MSR access failed: Disabled\n");
 		return;
-	case sld_warn:
-		pr_info("warning about user-space split_locks\n");
-		break;
-	case sld_fatal:
-		pr_info("sending SIGBUS on user-space split_locks\n");
-		break;
 	}
 
 	rdmsrl(MSR_TEST_CTRL, msr_test_ctrl_cache);
@@ -1061,7 +1059,9 @@ static void __init split_lock_setup(void)
 		return;
 	}
 
-	sld_state = state;
+	/* Restore the MSR to its cached value. */
+	wrmsrl(MSR_TEST_CTRL, msr_test_ctrl_cache);
+
 	setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
 }
 
@@ -1118,6 +1118,29 @@ bool handle_guest_split_lock(unsigned long ip)
 }
 EXPORT_SYMBOL_GPL(handle_guest_split_lock);
 
+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)
+		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;
+	wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
+}
+
 bool handle_user_split_lock(struct pt_regs *regs, long error_code)
 {
 	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
@@ -1126,6 +1149,21 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code)
 	return true;
 }
 
+void handle_bus_lock(struct pt_regs *regs)
+{
+	switch (sld_state) {
+	case sld_off:
+		break;
+	case sld_warn:
+		pr_warn_ratelimited("#DB: %s/%d took a bus_lock trap at address: 0x%lx\n",
+				    current->comm, current->pid, regs->ip);
+		break;
+	case sld_fatal:
+		force_sig_fault(SIGBUS, BUS_ADRALN, NULL);
+		break;
+	}
+}
+
 /*
  * This function is called only when switching between tasks with
  * different split-lock detection modes. It sets the MSR for the
@@ -1166,7 +1204,7 @@ static const struct x86_cpu_id split_lock_cpu_ids[] __initconst = {
 	{}
 };
 
-void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c)
+static void __init split_lock_setup(struct cpuinfo_x86 *c)
 {
 	const struct x86_cpu_id *m;
 	u64 ia32_core_caps;
@@ -1193,5 +1231,40 @@ void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c)
 	}
 
 	cpu_model_supports_sld = true;
-	split_lock_setup();
+	__split_lock_setup();
+}
+
+static void sld_state_show(void)
+{
+	if (!boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT) &&
+	    !boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		return;
+
+	switch (sld_state) {
+	case sld_off:
+		pr_info("disabled\n");
+		break;
+	case sld_warn:
+		if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+			pr_info("#AC: crashing the kernel on kernel split_locks and warning on user-space split_locks\n");
+		else if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT))
+			pr_info("#DB: warning on user-space bus_locks\n");
+		break;
+	case sld_fatal:
+		if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT)) {
+			pr_info("#AC: crashing the kernel on kernel split_locks and sending SIGBUS on user-space split_locks\n");
+		} else if (boot_cpu_has(X86_FEATURE_BUS_LOCK_DETECT)) {
+			pr_info("#DB: sending SIGBUS on user-space bus_locks%s\n",
+				boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) ?
+				" from non-WB" : "");
+		}
+		break;
+	}
+}
+
+void __init sld_setup(struct cpuinfo_x86 *c)
+{
+	split_lock_setup(c);
+	sld_state_setup();
+	sld_state_show();
 }
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 48881db8dca1..f577d07fbd43 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -978,6 +978,10 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
 		goto out_irq;
 	}
 
+	/* #DB for bus lock can only be triggered from userspace. */
+	if (dr6 & DR_BUS_LOCK)
+		handle_bus_lock(regs);
+
 	/* Add the virtual_dr6 bits for signals. */
 	dr6 |= current->thread.virtual_dr6;
 	if (dr6 & (DR_STEP | DR_TRAP_BITS) || icebp)