hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

This patch rebase the implementation of the breakpoints API on top of
perf events instances.

Each breakpoints are now perf events that handle the
register scheduling, thread/cpu attachment, etc..

The new layering is now made as follows:

       ptrace       kgdb      ftrace   perf syscall
          \          |          /         /
           \         |         /         /
                                        /
            Core breakpoint API        /
                                      /
                     |               /
                     |              /

              Breakpoints perf events

                     |
                     |

               Breakpoints PMU ---- Debug Register constraints handling
                                    (Part of core breakpoint API)
                     |
                     |

             Hardware debug registers

Reasons of this rewrite:

- Use the centralized/optimized pmu registers scheduling,
  implying an easier arch integration
- More powerful register handling: perf attributes (pinned/flexible
  events, exclusive/non-exclusive, tunable period, etc...)

Impact:

- New perf ABI: the hardware breakpoints counters
- Ptrace breakpoints setting remains tricky and still needs some per
  thread breakpoints references.

Todo (in the order):

- Support breakpoints perf counter events for perf tools (ie: implement
  perf_bpcounter_event())
- Support from perf tools

Changes in v2:

- Follow the perf "event " rename
- The ptrace regression have been fixed (ptrace breakpoint perf events
  weren't released when a task ended)
- Drop the struct hw_breakpoint and store generic fields in
  perf_event_attr.
- Separate core and arch specific headers, drop
  asm-generic/hw_breakpoint.h and create linux/hw_breakpoint.h
- Use new generic len/type for breakpoint
- Handle off case: when breakpoints api is not supported by an arch

Changes in v3:

- Fix broken CONFIG_KVM, we need to propagate the breakpoint api
  changes to kvm when we exit the guest and restore the bp registers
  to the host.

Changes in v4:

- Drop the hw_breakpoint_restore() stub as it is only used by KVM
- EXPORT_SYMBOL_GPL hw_breakpoint_restore() as KVM can be built as a
  module
- Restore the breakpoints unconditionally on kvm guest exit:
  TIF_DEBUG_THREAD doesn't anymore cover every cases of running
  breakpoints and vcpu->arch.switch_db_regs might not always be
  set when the guest used debug registers.
  (Waiting for a reliable optimization)

Changes in v5:

- Split-up the asm-generic/hw-breakpoint.h moving to
  linux/hw_breakpoint.h into a separate patch
- Optimize the breakpoints restoring while switching from kvm guest
  to host. We only want to restore the state if we have active
  breakpoints to the host, otherwise we don't care about messed-up
  address registers.
- Add asm/hw_breakpoint.h to Kbuild
- Fix bad breakpoint type in trace_selftest.c

Changes in v6:

- Fix wrong header inclusion in trace.h (triggered a build
  error with CONFIG_FTRACE_SELFTEST

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jan Kiszka <jan.kiszka@web.de>
Cc: Jiri Slaby <jirislaby@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Paul Mundt <lethal@linux-sh.org>

1 files changed, 255 insertions, 136 deletions

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 9316a9de4de..e622620790b 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -15,6 +15,7 @@
  *
  * Copyright (C) 2007 Alan Stern
  * Copyright (C) 2009 IBM Corporation
+ * Copyright (C) 2009 Frederic Weisbecker <fweisbec@gmail.com>
  */
 
 /*
@@ -22,6 +23,8 @@
  * using the CPU's debug registers.
  */
 
+#include <linux/perf_event.h>
+#include <linux/hw_breakpoint.h>
 #include <linux/irqflags.h>
 #include <linux/notifier.h>
 #include <linux/kallsyms.h>
@@ -38,26 +41,24 @@
 #include <asm/processor.h>
 #include <asm/debugreg.h>
 
-/* Unmasked kernel DR7 value */
-static unsigned long kdr7;
+/* Per cpu debug control register value */
+DEFINE_PER_CPU(unsigned long, dr7);
+
+/* Per cpu debug address registers values */
+static DEFINE_PER_CPU(unsigned long, cpu_debugreg[HBP_NUM]);
 
 /*
- * Masks for the bits corresponding to registers DR0 - DR3 in DR7 register.
- * Used to clear and verify the status of bits corresponding to DR0 - DR3
+ * Stores the breakpoints currently in use on each breakpoint address
+ * register for each cpus
  */
-static const unsigned long	dr7_masks[HBP_NUM] = {
-	0x000f0003,	/* LEN0, R/W0, G0, L0 */
-	0x00f0000c,	/* LEN1, R/W1, G1, L1 */
-	0x0f000030,	/* LEN2, R/W2, G2, L2 */
-	0xf00000c0	/* LEN3, R/W3, G3, L3 */
-};
+static DEFINE_PER_CPU(struct perf_event *, bp_per_reg[HBP_NUM]);
 
 
 /*
  * Encode the length, type, Exact, and Enable bits for a particular breakpoint
  * as stored in debug register 7.
  */
-static unsigned long encode_dr7(int drnum, unsigned int len, unsigned int type)
+unsigned long encode_dr7(int drnum, unsigned int len, unsigned int type)
 {
 	unsigned long bp_info;
 
@@ -68,64 +69,89 @@ static unsigned long encode_dr7(int drnum, unsigned int len, unsigned int type)
 	return bp_info;
 }
 
-void arch_update_kernel_hw_breakpoint(void *unused)
+/*
+ * Decode the length and type bits for a particular breakpoint as
+ * stored in debug register 7.  Return the "enabled" status.
+ */
+int decode_dr7(unsigned long dr7, int bpnum, unsigned *len, unsigned *type)
 {
-	struct hw_breakpoint *bp;
-	int i, cpu = get_cpu();
-	unsigned long temp_kdr7 = 0;
-
-	/* Don't allow debug exceptions while we update the registers */
-	set_debugreg(0UL, 7);
+	int bp_info = dr7 >> (DR_CONTROL_SHIFT + bpnum * DR_CONTROL_SIZE);
 
-	for (i = hbp_kernel_pos; i < HBP_NUM; i++) {
-		per_cpu(this_hbp_kernel[i], cpu) = bp = hbp_kernel[i];
-		if (bp) {
-			temp_kdr7 |= encode_dr7(i, bp->info.len, bp->info.type);
-			set_debugreg(bp->info.address, i);
-		}
-	}
+	*len = (bp_info & 0xc) | 0x40;
+	*type = (bp_info & 0x3) | 0x80;
 
-	/* No need to set DR6. Update the debug registers with kernel-space
-	 * breakpoint values from kdr7 and user-space requests from the
-	 * current process
-	 */
-	kdr7 = temp_kdr7;
-	set_debugreg(kdr7 | current->thread.debugreg7, 7);
-	put_cpu();
+	return (dr7 >> (bpnum * DR_ENABLE_SIZE)) & 0x3;
 }
 
 /*
- * Install the thread breakpoints in their debug registers.
+ * Install a perf counter breakpoint.
+ *
+ * We seek a free debug address register and use it for this
+ * breakpoint. Eventually we enable it in the debug control register.
+ *
+ * Atomic: we hold the counter->ctx->lock and we only handle variables
+ * and registers local to this cpu.
  */
-void arch_install_thread_hw_breakpoint(struct task_struct *tsk)
+int arch_install_hw_breakpoint(struct perf_event *bp)
 {
-	struct thread_struct *thread = &(tsk->thread);
-
-	switch (hbp_kernel_pos) {
-	case 4:
-		set_debugreg(thread->debugreg[3], 3);
-	case 3:
-		set_debugreg(thread->debugreg[2], 2);
-	case 2:
-		set_debugreg(thread->debugreg[1], 1);
-	case 1:
-		set_debugreg(thread->debugreg[0], 0);
-	default:
-		break;
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	unsigned long *dr7;
+	int i;
+
+	for (i = 0; i < HBP_NUM; i++) {
+		struct perf_event **slot = &__get_cpu_var(bp_per_reg[i]);
+
+		if (!*slot) {
+			*slot = bp;
+			break;
+		}
 	}
 
-	/* No need to set DR6 */
-	set_debugreg((kdr7 | thread->debugreg7), 7);
+	if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
+		return -EBUSY;
+
+	set_debugreg(info->address, i);
+	__get_cpu_var(cpu_debugreg[i]) = info->address;
+
+	dr7 = &__get_cpu_var(dr7);
+	*dr7 |= encode_dr7(i, info->len, info->type);
+
+	set_debugreg(*dr7, 7);
+
+	return 0;
 }
 
 /*
- * Install the debug register values for just the kernel, no thread.
+ * Uninstall the breakpoint contained in the given counter.
+ *
+ * First we search the debug address register it uses and then we disable
+ * it.
+ *
+ * Atomic: we hold the counter->ctx->lock and we only handle variables
+ * and registers local to this cpu.
  */
-void arch_uninstall_thread_hw_breakpoint(void)
+void arch_uninstall_hw_breakpoint(struct perf_event *bp)
 {
-	/* Clear the user-space portion of debugreg7 by setting only kdr7 */
-	set_debugreg(kdr7, 7);
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	unsigned long *dr7;
+	int i;
+
+	for (i = 0; i < HBP_NUM; i++) {
+		struct perf_event **slot = &__get_cpu_var(bp_per_reg[i]);
+
+		if (*slot == bp) {
+			*slot = NULL;
+			break;
+		}
+	}
+
+	if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
+		return;
 
+	dr7 = &__get_cpu_var(dr7);
+	*dr7 &= ~encode_dr7(i, info->len, info->type);
+
+	set_debugreg(*dr7, 7);
 }
 
 static int get_hbp_len(u8 hbp_len)
@@ -133,17 +159,17 @@ static int get_hbp_len(u8 hbp_len)
 	unsigned int len_in_bytes = 0;
 
 	switch (hbp_len) {
-	case HW_BREAKPOINT_LEN_1:
+	case X86_BREAKPOINT_LEN_1:
 		len_in_bytes = 1;
 		break;
-	case HW_BREAKPOINT_LEN_2:
+	case X86_BREAKPOINT_LEN_2:
 		len_in_bytes = 2;
 		break;
-	case HW_BREAKPOINT_LEN_4:
+	case X86_BREAKPOINT_LEN_4:
 		len_in_bytes = 4;
 		break;
 #ifdef CONFIG_X86_64
-	case HW_BREAKPOINT_LEN_8:
+	case X86_BREAKPOINT_LEN_8:
 		len_in_bytes = 8;
 		break;
 #endif
@@ -178,67 +204,146 @@ static int arch_check_va_in_kernelspace(unsigned long va, u8 hbp_len)
 /*
  * Store a breakpoint's encoded address, length, and type.
  */
-static int arch_store_info(struct hw_breakpoint *bp, struct task_struct *tsk)
+static int arch_store_info(struct perf_event *bp)
 {
-	/*
-	 * User-space requests will always have the address field populated
-	 * Symbol names from user-space are rejected
-	 */
-	if (tsk && bp->info.name)
-		return -EINVAL;
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
 	/*
 	 * For kernel-addresses, either the address or symbol name can be
 	 * specified.
 	 */
-	if (bp->info.name)
-		bp->info.address = (unsigned long)
-					kallsyms_lookup_name(bp->info.name);
-	if (bp->info.address)
+	if (info->name)
+		info->address = (unsigned long)
+				kallsyms_lookup_name(info->name);
+	if (info->address)
 		return 0;
+
 	return -EINVAL;
 }
 
-/*
- * Validate the arch-specific HW Breakpoint register settings
- */
-int arch_validate_hwbkpt_settings(struct hw_breakpoint *bp,
-						struct task_struct *tsk)
+int arch_bp_generic_fields(int x86_len, int x86_type,
+			   int *gen_len, int *gen_type)
 {
-	unsigned int align;
-	int ret = -EINVAL;
+	/* Len */
+	switch (x86_len) {
+	case X86_BREAKPOINT_LEN_1:
+		*gen_len = HW_BREAKPOINT_LEN_1;
+		break;
+	case X86_BREAKPOINT_LEN_2:
+		*gen_len = HW_BREAKPOINT_LEN_2;
+		break;
+	case X86_BREAKPOINT_LEN_4:
+		*gen_len = HW_BREAKPOINT_LEN_4;
+		break;
+#ifdef CONFIG_X86_64
+	case X86_BREAKPOINT_LEN_8:
+		*gen_len = HW_BREAKPOINT_LEN_8;
+		break;
+#endif
+	default:
+		return -EINVAL;
+	}
 
-	switch (bp->info.type) {
-	/*
-	 * Ptrace-refactoring code
-	 * For now, we'll allow instruction breakpoint only for user-space
-	 * addresses
-	 */
-	case HW_BREAKPOINT_EXECUTE:
-		if ((!arch_check_va_in_userspace(bp->info.address,
-							bp->info.len)) &&
-			bp->info.len != HW_BREAKPOINT_LEN_EXECUTE)
-			return ret;
+	/* Type */
+	switch (x86_type) {
+	case X86_BREAKPOINT_EXECUTE:
+		*gen_type = HW_BREAKPOINT_X;
 		break;
-	case HW_BREAKPOINT_WRITE:
+	case X86_BREAKPOINT_WRITE:
+		*gen_type = HW_BREAKPOINT_W;
 		break;
-	case HW_BREAKPOINT_RW:
+	case X86_BREAKPOINT_RW:
+		*gen_type = HW_BREAKPOINT_W | HW_BREAKPOINT_R;
 		break;
 	default:
-		return ret;
+		return -EINVAL;
 	}
 
-	switch (bp->info.len) {
+	return 0;
+}
+
+
+static int arch_build_bp_info(struct perf_event *bp)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+
+	info->address = bp->attr.bp_addr;
+
+	/* Len */
+	switch (bp->attr.bp_len) {
 	case HW_BREAKPOINT_LEN_1:
-		align = 0;
+		info->len = X86_BREAKPOINT_LEN_1;
 		break;
 	case HW_BREAKPOINT_LEN_2:
-		align = 1;
+		info->len = X86_BREAKPOINT_LEN_2;
 		break;
 	case HW_BREAKPOINT_LEN_4:
-		align = 3;
+		info->len = X86_BREAKPOINT_LEN_4;
 		break;
 #ifdef CONFIG_X86_64
 	case HW_BREAKPOINT_LEN_8:
+		info->len = X86_BREAKPOINT_LEN_8;
+		break;
+#endif
+	default:
+		return -EINVAL;
+	}
+
+	/* Type */
+	switch (bp->attr.bp_type) {
+	case HW_BREAKPOINT_W:
+		info->type = X86_BREAKPOINT_WRITE;
+		break;
+	case HW_BREAKPOINT_W | HW_BREAKPOINT_R:
+		info->type = X86_BREAKPOINT_RW;
+		break;
+	case HW_BREAKPOINT_X:
+		info->type = X86_BREAKPOINT_EXECUTE;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+}
+/*
+ * Validate the arch-specific HW Breakpoint register settings
+ */
+int arch_validate_hwbkpt_settings(struct perf_event *bp,
+				  struct task_struct *tsk)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	unsigned int align;
+	int ret;
+
+
+	ret = arch_build_bp_info(bp);
+	if (ret)
+		return ret;
+
+	ret = -EINVAL;
+
+	if (info->type == X86_BREAKPOINT_EXECUTE)
+		/*
+		 * Ptrace-refactoring code
+		 * For now, we'll allow instruction breakpoint only for user-space
+		 * addresses
+		 */
+		if ((!arch_check_va_in_userspace(info->address, info->len)) &&
+			info->len != X86_BREAKPOINT_EXECUTE)
+			return ret;
+
+	switch (info->len) {
+	case X86_BREAKPOINT_LEN_1:
+		align = 0;
+		break;
+	case X86_BREAKPOINT_LEN_2:
+		align = 1;
+		break;
+	case X86_BREAKPOINT_LEN_4:
+		align = 3;
+		break;
+#ifdef CONFIG_X86_64
+	case X86_BREAKPOINT_LEN_8:
 		align = 7;
 		break;
 #endif
@@ -246,8 +351,8 @@ int arch_validate_hwbkpt_settings(struct hw_breakpoint *bp,
 		return ret;
 	}
 
-	if (bp->triggered)
-		ret = arch_store_info(bp, tsk);
+	if (bp->callback)
+		ret = arch_store_info(bp);
 
 	if (ret < 0)
 		return ret;
@@ -255,44 +360,47 @@ int arch_validate_hwbkpt_settings(struct hw_breakpoint *bp,
 	 * Check that the low-order bits of the address are appropriate
 	 * for the alignment implied by len.
 	 */
-	if (bp->info.address & align)
+	if (info->address & align)
 		return -EINVAL;
 
 	/* Check that the virtual address is in the proper range */
 	if (tsk) {
-		if (!arch_check_va_in_userspace(bp->info.address, bp->info.len))
+		if (!arch_check_va_in_userspace(info->address, info->len))
 			return -EFAULT;
 	} else {
-		if (!arch_check_va_in_kernelspace(bp->info.address,
-								bp->info.len))
+		if (!arch_check_va_in_kernelspace(info->address, info->len))
 			return -EFAULT;
 	}
+
 	return 0;
 }
 
-void arch_update_user_hw_breakpoint(int pos, struct task_struct *tsk)
+/*
+ * Release the user breakpoints used by ptrace
+ */
+void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
 {
-	struct thread_struct *thread = &(tsk->thread);
-	struct hw_breakpoint *bp = thread->hbp[pos];
-
-	thread->debugreg7 &= ~dr7_masks[pos];
-	if (bp) {
-		thread->debugreg[pos] = bp->info.address;
-		thread->debugreg7 |= encode_dr7(pos, bp->info.len,
-							bp->info.type);
-	} else
-		thread->debugreg[pos] = 0;
+	int i;
+	struct thread_struct *t = &tsk->thread;
+
+	for (i = 0; i < HBP_NUM; i++) {
+		unregister_hw_breakpoint(t->ptrace_bps[i]);
+		t->ptrace_bps[i] = NULL;
+	}
 }
 
-void arch_flush_thread_hw_breakpoint(struct task_struct *tsk)
+#ifdef CONFIG_KVM
+void hw_breakpoint_restore(void)
 {
-	int i;
-	struct thread_struct *thread = &(tsk->thread);
-
-	thread->debugreg7 = 0;
-	for (i = 0; i < HBP_NUM; i++)
-		thread->debugreg[i] = 0;
+	set_debugreg(__get_cpu_var(cpu_debugreg[0]), 0);
+	set_debugreg(__get_cpu_var(cpu_debugreg[1]), 1);
+	set_debugreg(__get_cpu_var(cpu_debugreg[2]), 2);
+	set_debugreg(__get_cpu_var(cpu_debugreg[3]), 3);
+	set_debugreg(current->thread.debugreg6, 6);
+	set_debugreg(__get_cpu_var(dr7), 7);
 }
+EXPORT_SYMBOL_GPL(hw_breakpoint_restore);
+#endif
 
 /*
  * Handle debug exception notifications.
@@ -313,7 +421,7 @@ void arch_flush_thread_hw_breakpoint(struct task_struct *tsk)
 static int __kprobes hw_breakpoint_handler(struct die_args *args)
 {
 	int i, cpu, rc = NOTIFY_STOP;
-	struct hw_breakpoint *bp;
+	struct perf_event *bp;
 	unsigned long dr7, dr6;
 	unsigned long *dr6_p;
 
@@ -325,10 +433,6 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
 	if ((dr6 & DR_TRAP_BITS) == 0)
 		return NOTIFY_DONE;
 
-	/* Lazy debug register switching */
-	if (!test_tsk_thread_flag(current, TIF_DEBUG))
-		arch_uninstall_thread_hw_breakpoint();
-
 	get_debugreg(dr7, 7);
 	/* Disable breakpoints during exception handling */
 	set_debugreg(0UL, 7);
@@ -344,17 +448,18 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
 	for (i = 0; i < HBP_NUM; ++i) {
 		if (likely(!(dr6 & (DR_TRAP0 << i))))
 			continue;
+
 		/*
-		 * Find the corresponding hw_breakpoint structure and
-		 * invoke its triggered callback.
+		 * The counter may be concurrently released but that can only
+		 * occur from a call_rcu() path. We can then safely fetch
+		 * the breakpoint, use its callback, touch its counter
+		 * while we are in an rcu_read_lock() path.
 		 */
-		if (i >= hbp_kernel_pos)
-			bp = per_cpu(this_hbp_kernel[i], cpu);
-		else {
-			bp = current->thread.hbp[i];
-			if (bp)
-				rc = NOTIFY_DONE;
-		}
+		rcu_read_lock();
+
+		bp = per_cpu(bp_per_reg[i], cpu);
+		if (bp)
+			rc = NOTIFY_DONE;
 		/*
 		 * Reset the 'i'th TRAP bit in dr6 to denote completion of
 		 * exception handling
@@ -362,19 +467,23 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
 		(*dr6_p) &= ~(DR_TRAP0 << i);
 		/*
 		 * bp can be NULL due to lazy debug register switching
-		 * or due to the delay between updates of hbp_kernel_pos
-		 * and this_hbp_kernel.
+		 * or due to concurrent perf counter removing.
 		 */
-		if (!bp)
-			continue;
+		if (!bp) {
+			rcu_read_unlock();
+			break;
+		}
+
+		(bp->callback)(bp, args->regs);
 
-		(bp->triggered)(bp, args->regs);
+		rcu_read_unlock();
 	}
 	if (dr6 & (~DR_TRAP_BITS))
 		rc = NOTIFY_DONE;
 
 	set_debugreg(dr7, 7);
 	put_cpu();
+
 	return rc;
 }
 
@@ -389,3 +498,13 @@ int __kprobes hw_breakpoint_exceptions_notify(
 
 	return hw_breakpoint_handler(data);
 }
+
+void hw_breakpoint_pmu_read(struct perf_event *bp)
+{
+	/* TODO */
+}
+
+void hw_breakpoint_pmu_unthrottle(struct perf_event *bp)
+{
+	/* TODO */
+}