[PATCH v15] x86/split_lock: Enable split lock detection by kernel

From: "Luck, Tony" <tony.luck@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
	"Christopherson, Sean J" <sean.j.christopherson@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	"Yu, Fenghua" <fenghua.yu@intel.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	H Peter Anvin <hpa@zytor.com>, "Raj, Ashok" <ashok.raj@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: [PATCH v15] x86/split_lock: Enable split lock detection by kernel
Date: Fri, 24 Jan 2020 18:47:27 -0800	[thread overview]
Message-ID: <20200125024727.GA32483@agluck-desk2.amr.corp.intel.com> (raw)
In-Reply-To: <87h80kmta4.fsf@nanos.tec.linutronix.de>

From: Peter Zijlstra <peterz@infradead.org>

A split-lock occurs when an atomic instruction operates on data
that spans two cache lines. In order to maintain atomicity the
core takes a global bus lock.

This is typically >1000 cycles slower than an atomic operation
within a cache line. It also disrupts performance on other cores
(which must wait for the bus lock to be released before their
memory operations can complete). For real-time systems this may
mean missing deadlines. For other systems it may just be very
annoying.

Some CPUs have the capability to raise an #AC trap when a
split lock is attempted.

Provide a command line option to give the user choices on how
to handle this. split_lock_detect=
	off	- not enabled (no traps for split locks)
	warn	- warn once when an application does a
		  split lock, but allow it to continue
		  running.
	fatal	- Send SIGBUS to applications that cause split lock

On systems that support split lock detection the default is "warn". Note
that if the kernel hits a split lock in any mode other than "off" it
will OOPs.

One implementation wrinkle is that the MSR to control the
split lock detection is per-core, not per thread. This might
result in some short lived races on HT systems in "warn" mode
if Linux tries to enable on one thread while disabling on
the other. Race analysis by Sean Christopherson:

  - Toggling of split-lock is only done in "warn" mode.  Worst case
    scenario of a race is that a misbehaving task will generate multiple
    #AC exceptions on the same instruction.  And this race will only occur
    if both siblings are running tasks that generate split-lock #ACs, e.g.
    a race where sibling threads are writing different values will only
    occur if CPUx is disabling split-lock after an #AC and CPUy is
    re-enabling split-lock after *its* previous task generated an #AC.
  - Transitioning between modes at runtime isn't supported and disabling
    is tracked per task, so hardware will always reach a steady state that
    matches the configured mode.  I.e. split-lock is guaranteed to be
    enabled in hardware once all _TIF_SLD threads have been scheduled out.

Co-developed-by: Fenghua Yu <fenghua.yu@intel.com>
Co-developed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>
---

tglx> Other than those details, I really like this approach.

Thanks for the review. Here is V15 with all your V14 comments addressed.

I did find something with a new test. Applications that hit a
split lock warn as expected. But if they sleep before they hit
a new split lock, we get another warning. This is may be because
I messed up when fixing a PeterZ typo in the untested patch.
But I think there may have been bigger problems.

Context switch in V14 code did: 

       if (tifp & _TIF_SLD)
               switch_to_sld(prev_p);

void switch_to_sld(struct task_struct *prev)
{
       __sld_msr_set(true);
       clear_tsk_thread_flag(prev, TIF_SLD);
}

Which re-enables split lock checking for the next process to run. But
mysteriously clears the TIF_SLD bit on the previous task.

I think we need to consider TIF_SLD state of both previous and next
process when deciding what to do with the MSR. Three cases:

1) If they are both the same, leave the MSR alone it is (probably) right (modulo
   the other thread having messed with it).
2) Next process has _TIF_SLD set ... disable checking
3) Next process doesn't have _TIF_SLD set ... enable checking

So please look closely at the new version of switch_to_sld() which is
now called unconditonally on every switch ... but commonly will do
nothing.

 .../admin-guide/kernel-parameters.txt         |  18 ++
 arch/x86/include/asm/cpu.h                    |  12 ++
 arch/x86/include/asm/cpufeatures.h            |   2 +
 arch/x86/include/asm/msr-index.h              |   9 +
 arch/x86/include/asm/thread_info.h            |   6 +-
 arch/x86/kernel/cpu/common.c                  |   2 +
 arch/x86/kernel/cpu/intel.c                   | 177 ++++++++++++++++++
 arch/x86/kernel/process.c                     |   2 +
 arch/x86/kernel/traps.c                       |  24 ++-
 9 files changed, 248 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7f1e2f327e43..27f61d44a37f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3207,6 +3207,24 @@
 
 	nosoftlockup	[KNL] Disable the soft-lockup detector.
 
+	split_lock_detect=
+			[X86] Enable split lock detection
+
+			When enabled (and if hardware support is present), atomic
+			instructions that access data across cache line
+			boundaries will result in an alignment check exception.
+
+			off	- not enabled
+
+			warn	- the kernel will emit rate limited warnings
+				  about applications triggering the #AC exception
+
+			fatal	- the kernel will SIGBUS applications that
+				  trigger the #AC exception.
+
+			For any more other than 'off' the kernel will die if
+			it (or firmware) will trigger #AC.
+
 	nosync		[HW,M68K] Disables sync negotiation for all devices.
 
 	nowatchdog	[KNL] Disable both lockup detectors, i.e.
diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index adc6cc86b062..2dede2bbb7cf 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -40,4 +40,16 @@ int mwait_usable(const struct cpuinfo_x86 *);
 unsigned int x86_family(unsigned int sig);
 unsigned int x86_model(unsigned int sig);
 unsigned int x86_stepping(unsigned int sig);
+#ifdef CONFIG_CPU_SUP_INTEL
+extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
+extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
+extern void switch_to_sld(struct task_struct *, struct task_struct *);
+#else
+static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
+static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+{
+	return false;
+}
+static inline void switch_to_sld(struct task_struct *prev, struct stack *next) {}
+#endif
 #endif /* _ASM_X86_CPU_H */
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index f3327cb56edf..cd56ad5d308e 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -285,6 +285,7 @@
 #define X86_FEATURE_CQM_MBM_LOCAL	(11*32+ 3) /* LLC Local MBM monitoring */
 #define X86_FEATURE_FENCE_SWAPGS_USER	(11*32+ 4) /* "" LFENCE in user entry SWAPGS path */
 #define X86_FEATURE_FENCE_SWAPGS_KERNEL	(11*32+ 5) /* "" LFENCE in kernel entry SWAPGS path */
+#define X86_FEATURE_SPLIT_LOCK_DETECT	(11*32+ 6) /* #AC for split lock */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
 #define X86_FEATURE_AVX512_BF16		(12*32+ 5) /* AVX512 BFLOAT16 instructions */
@@ -367,6 +368,7 @@
 #define X86_FEATURE_INTEL_STIBP		(18*32+27) /* "" Single Thread Indirect Branch Predictors */
 #define X86_FEATURE_FLUSH_L1D		(18*32+28) /* Flush L1D cache */
 #define X86_FEATURE_ARCH_CAPABILITIES	(18*32+29) /* IA32_ARCH_CAPABILITIES MSR (Intel) */
+#define X86_FEATURE_CORE_CAPABILITIES	(18*32+30) /* "" IA32_CORE_CAPABILITIES MSR */
 #define X86_FEATURE_SPEC_CTRL_SSBD	(18*32+31) /* "" Speculative Store Bypass Disable */
 
 /*
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index ebe1685e92dd..8821697a7549 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -41,6 +41,10 @@
 
 /* Intel MSRs. Some also available on other CPUs */
 
+#define MSR_TEST_CTRL				0x00000033
+#define MSR_TEST_CTRL_SPLIT_LOCK_DETECT_BIT	29
+#define MSR_TEST_CTRL_SPLIT_LOCK_DETECT		BIT(MSR_TEST_CTRL_SPLIT_LOCK_DETECT_BIT)
+
 #define MSR_IA32_SPEC_CTRL		0x00000048 /* Speculation Control */
 #define SPEC_CTRL_IBRS			BIT(0)	   /* Indirect Branch Restricted Speculation */
 #define SPEC_CTRL_STIBP_SHIFT		1	   /* Single Thread Indirect Branch Predictor (STIBP) bit */
@@ -70,6 +74,11 @@
  */
 #define MSR_IA32_UMWAIT_CONTROL_TIME_MASK	(~0x03U)
 
+/* Abbreviated from Intel SDM name IA32_CORE_CAPABILITIES */
+#define MSR_IA32_CORE_CAPS			  0x000000cf
+#define MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT_BIT  5
+#define MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT	  BIT(MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT_BIT)
+
 #define MSR_PKG_CST_CONFIG_CONTROL	0x000000e2
 #define NHM_C3_AUTO_DEMOTE		(1UL << 25)
 #define NHM_C1_AUTO_DEMOTE		(1UL << 26)
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index cf4327986e98..e0d12517f348 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -92,6 +92,7 @@ struct thread_info {
 #define TIF_NOCPUID		15	/* CPUID is not accessible in userland */
 #define TIF_NOTSC		16	/* TSC is not accessible in userland */
 #define TIF_IA32		17	/* IA32 compatibility process */
+#define TIF_SLD			18	/* Restore split lock detection on context switch */
 #define TIF_NOHZ		19	/* in adaptive nohz mode */
 #define TIF_MEMDIE		20	/* is terminating due to OOM killer */
 #define TIF_POLLING_NRFLAG	21	/* idle is polling for TIF_NEED_RESCHED */
@@ -122,6 +123,7 @@ struct thread_info {
 #define _TIF_NOCPUID		(1 << TIF_NOCPUID)
 #define _TIF_NOTSC		(1 << TIF_NOTSC)
 #define _TIF_IA32		(1 << TIF_IA32)
+#define _TIF_SLD		(1 << TIF_SLD)
 #define _TIF_NOHZ		(1 << TIF_NOHZ)
 #define _TIF_POLLING_NRFLAG	(1 << TIF_POLLING_NRFLAG)
 #define _TIF_IO_BITMAP		(1 << TIF_IO_BITMAP)
@@ -158,9 +160,9 @@ struct thread_info {
 
 #ifdef CONFIG_X86_IOPL_IOPERM
 # define _TIF_WORK_CTXSW_PREV	(_TIF_WORK_CTXSW| _TIF_USER_RETURN_NOTIFY | \
-				 _TIF_IO_BITMAP)
+				 _TIF_IO_BITMAP | _TIF_SLD)
 #else
-# define _TIF_WORK_CTXSW_PREV	(_TIF_WORK_CTXSW| _TIF_USER_RETURN_NOTIFY)
+# define _TIF_WORK_CTXSW_PREV	(_TIF_WORK_CTXSW| _TIF_USER_RETURN_NOTIFY | _TIF_SLD)
 #endif
 
 #define _TIF_WORK_CTXSW_NEXT	(_TIF_WORK_CTXSW)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 86b8241c8209..adb2f639f388 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1242,6 +1242,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	cpu_set_bug_bits(c);
 
+	cpu_set_core_cap_bits(c);
+
 	fpu__init_system(c);
 
 #ifdef CONFIG_X86_32
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 57473e2c0869..d9842c64e5af 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -19,6 +19,8 @@
 #include <asm/microcode_intel.h>
 #include <asm/hwcap2.h>
 #include <asm/elf.h>
+#include <asm/cpu_device_id.h>
+#include <asm/cmdline.h>
 
 #ifdef CONFIG_X86_64
 #include <linux/topology.h>
@@ -31,6 +33,20 @@
 #include <asm/apic.h>
 #endif
 
+enum split_lock_detect_state {
+	sld_off = 0,
+	sld_warn,
+	sld_fatal,
+};
+
+/*
+ * 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, and
+ * then check to see if there is a command line override.
+ */
+static enum split_lock_detect_state sld_state = sld_off;
+
 /*
  * Just in case our CPU detection goes bad, or you have a weird system,
  * allow a way to override the automatic disabling of MPX.
@@ -606,6 +622,8 @@ static void init_intel_misc_features(struct cpuinfo_x86 *c)
 	wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
 }
 
+static void split_lock_init(void);
+
 static void init_intel(struct cpuinfo_x86 *c)
 {
 	early_init_intel(c);
@@ -720,6 +738,8 @@ static void init_intel(struct cpuinfo_x86 *c)
 		tsx_enable();
 	if (tsx_ctrl_state == TSX_CTRL_DISABLE)
 		tsx_disable();
+
+	split_lock_init();
 }
 
 #ifdef CONFIG_X86_32
@@ -981,3 +1001,160 @@ static const struct cpu_dev intel_cpu_dev = {
 };
 
 cpu_dev_register(intel_cpu_dev);
+
+#undef pr_fmt
+#define pr_fmt(fmt) "x86/split lock detection: " fmt
+
+static const struct {
+	const char			*option;
+	enum split_lock_detect_state	state;
+} sld_options[] __initconst = {
+	{ "off",	sld_off   },
+	{ "warn",	sld_warn  },
+	{ "fatal",	sld_fatal },
+};
+
+static inline bool match_option(const char *arg, int arglen, const char *opt)
+{
+	int len = strlen(opt);
+
+	return len == arglen && !strncmp(arg, opt, len);
+}
+
+static void __init split_lock_setup(void)
+{
+	char arg[20];
+	int i, ret;
+
+	sld_state = sld_warn;
+	setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
+
+	ret = cmdline_find_option(boot_command_line, "split_lock_detect",
+				  arg, sizeof(arg));
+	if (ret < 0)
+		goto print;
+
+	for (i = 0; i < ARRAY_SIZE(sld_options); i++) {
+		if (match_option(arg, ret, sld_options[i].option)) {
+			sld_state = sld_options[i].state;
+			break;
+		}
+	}
+
+print:
+	switch(sld_state) {
+	case sld_off:
+		pr_info("disabled\n");
+		break;
+
+	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;
+	}
+}
+
+/*
+ * Locking is not required at the moment because only bit 29 of this
+ * MSR is implemented and locking would not prevent that the operation
+ * of one thread is immediately undone by the sibling thread.
+ */
+
+static bool __sld_msr_set(bool on)
+{
+	u64 test_ctrl_val;
+
+	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
+		return false;
+
+	if (on)
+		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+	else
+		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
+
+	return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val);
+}
+
+static void split_lock_init(void)
+{
+	if (sld_state == sld_off)
+		return;
+
+	if (__sld_msr_set(true))
+		return;
+
+	/*
+	 * If this is anything other than the boot-cpu, you've done
+	 * funny things and you get to keep whatever pieces.
+	 */
+	pr_warn("MSR fail -- disabled\n");
+	__sld_msr_set(sld_off);
+	sld_state = sld_off;
+}
+
+bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+{
+	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
+		return false;
+
+	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
+		 current->comm, current->pid, regs->ip);
+
+	/*
+	 * Disable the split lock detection for this task so it can make
+	 * progress and set TIF_SLD so the detection is reenabled via
+	 * switch_to_sld() when the task is scheduled out.
+	 */
+	__sld_msr_set(false);
+	set_tsk_thread_flag(current, TIF_SLD);
+	return true;
+}
+
+void switch_to_sld(struct task_struct *prev, struct task_struct *next)
+{
+	bool prevflag = test_tsk_thread_flag(prev, TIF_SLD);
+	bool nextflag = test_tsk_thread_flag(next, TIF_SLD);
+
+	/*
+	 * If we are switching between tasks that have the same
+	 * need for split lock checking, then the MSR is (probably)
+	 * right (modulo the other thread messing with it.
+	 * Otherwise look at whether the new task needs split
+	 * lock enabled.
+	 */
+	if (prevflag != nextflag)
+		__sld_msr_set(nextflag);
+}
+
+#define SPLIT_LOCK_CPU(model) {X86_VENDOR_INTEL, 6, model, X86_FEATURE_ANY}
+
+/*
+ * The following processors have split lock detection feature. But since they
+ * don't have MSR IA32_CORE_CAPABILITIES, the feature cannot be enumerated by
+ * the MSR. So enumerate the feature by family and model on these processors.
+ */
+static const struct x86_cpu_id split_lock_cpu_ids[] __initconst = {
+	SPLIT_LOCK_CPU(INTEL_FAM6_ICELAKE_X),
+	SPLIT_LOCK_CPU(INTEL_FAM6_ICELAKE_L),
+	{}
+};
+
+void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c)
+{
+	u64 ia32_core_caps = 0;
+
+	if (cpu_has(c, X86_FEATURE_CORE_CAPABILITIES)) {
+		/* Enumerate features reported in IA32_CORE_CAPABILITIES MSR. */
+		rdmsrl(MSR_IA32_CORE_CAPS, ia32_core_caps);
+	} else if (!boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
+		/* Enumerate split lock detection by family and model. */
+		if (x86_match_cpu(split_lock_cpu_ids))
+			ia32_core_caps |= MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT;
+	}
+
+	if (ia32_core_caps & MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT)
+		split_lock_setup();
+}
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 839b5244e3b7..b34d359c4e39 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -650,6 +650,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
 		/* Enforce MSR update to ensure consistent state */
 		__speculation_ctrl_update(~tifn, tifn);
 	}
+
+	switch_to_sld(prev_p, next_p);
 }
 
 /*
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9e6f822922a3..884e8e59dafd 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -46,6 +46,7 @@
 #include <asm/traps.h>
 #include <asm/desc.h>
 #include <asm/fpu/internal.h>
+#include <asm/cpu.h>
 #include <asm/cpu_entry_area.h>
 #include <asm/mce.h>
 #include <asm/fixmap.h>
@@ -244,7 +245,6 @@ do_trap(int trapnr, int signr, char *str, struct pt_regs *regs,
 {
 	struct task_struct *tsk = current;
 
-
 	if (!do_trap_no_signal(tsk, trapnr, str, regs, error_code))
 		return;
 
@@ -290,9 +290,29 @@ 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)
+{
+	const char str[] = "alignment check";
+
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+
+	if (notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_AC, SIGBUS) == NOTIFY_STOP)
+		return;
+
+	if (!user_mode(regs))
+		die("Split lock detected\n", regs, error_code);
+
+	local_irq_enable();
+
+	if (handle_user_split_lock(regs, error_code))
+		return;
+
+	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
+		error_code, BUS_ADRALN, NULL);
+}
+
 #ifdef CONFIG_VMAP_STACK
 __visible void __noreturn handle_stack_overflow(const char *message,
 						struct pt_regs *regs,
-- 
2.21.1

