[RFC][PATCH] perf, intel: Don't touch MSR_IA32_DEBUGCTLMSR from NMI context

[Peter Zijlstra <peterz@infradead.org>]

Oleg and Sebastian found that touching MSR_IA32_DEBUGCTLMSR from NMI
context is problematic since the only way to change the various
unrelated bits in there is:

  debugctl = get_debugctlmsr()
  /* frob flags in debugctl */
  update_debugctlmsr(debugctl);

Which is entirely unsafe if we prod at the MSR from NMI context.

In particular the path that is responsible is:

  x86_pmu_handle_irq() (NMI handler)
    x86_pmu_stop()
      x86_pmu.disable -> intel_pmu_disable_event()
        intel_pmu_lbr_disable()
          __intel_pmu_lbr_disable()
            wrmsrl(MSR_IA32_DEBUGCTLMSR,... );

So remove intel_pmu_lbr_{en,dis}able() from
intel_pmu_{en,dis}able_event() and stick them in
intel_{get,put}_event_constraints() which are only ever called from
regular contexts.

We combine intel_pmu_needs_lbr_smpl(), which tells us if the events
wants LBR data, with event->hw.branch_reg.alloc, which given
intel_shared_regs_constraints() is set if our event got the shared
resource, to give us a correctly balanced condition in
{get,put}_event_constraints() for intel_pmu_lbr_{en,dis}able().

Also change core_pmu to only use x86_get_event_constraints since it
doesn't support any of the fancy DS (BTS,PEBS), LBR and OFFCORE features
that make up intel_{get,put}_event_constraints. 

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 arch/x86/kernel/cpu/perf_event_intel.c | 48 ++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
index 0d3d63a..ef4cd36 100644
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -821,6 +821,11 @@ static inline bool intel_pmu_needs_lbr_smpl(struct perf_event *event)
 	return false;
 }
 
+static inline bool intel_pmu_has_lbr(struct perf_event *event)
+{
+	return intel_pmu_needs_lbr_smpl(event) && event->hw.branch_reg.alloc;
+}
+
 static void intel_pmu_disable_all(void)
 {
 	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
@@ -975,13 +980,6 @@ static void intel_pmu_disable_event(struct perf_event *event)
 	cpuc->intel_ctrl_guest_mask &= ~(1ull << hwc->idx);
 	cpuc->intel_ctrl_host_mask &= ~(1ull << hwc->idx);
 
-	/*
-	 * must disable before any actual event
-	 * because any event may be combined with LBR
-	 */
-	if (intel_pmu_needs_lbr_smpl(event))
-		intel_pmu_lbr_disable(event);
-
 	if (unlikely(hwc->config_base == MSR_ARCH_PERFMON_FIXED_CTR_CTRL)) {
 		intel_pmu_disable_fixed(hwc);
 		return;
@@ -1036,12 +1034,6 @@ static void intel_pmu_enable_event(struct perf_event *event)
 		intel_pmu_enable_bts(hwc->config);
 		return;
 	}
-	/*
-	 * must enabled before any actual event
-	 * because any event may be combined with LBR
-	 */
-	if (intel_pmu_needs_lbr_smpl(event))
-		intel_pmu_lbr_enable(event);
 
 	if (event->attr.exclude_host)
 		cpuc->intel_ctrl_guest_mask |= (1ull << hwc->idx);
@@ -1382,17 +1374,28 @@ intel_get_event_constraints(struct cpu_hw_events *cpuc, struct perf_event *event
 
 	c = intel_bts_constraints(event);
 	if (c)
-		return c;
+		goto got_constraint;
 
 	c = intel_pebs_constraints(event);
 	if (c)
-		return c;
+		goto got_constraint;
 
 	c = intel_shared_regs_constraints(cpuc, event);
 	if (c)
-		return c;
+		goto got_constraint;
+
+	c = x86_get_event_constraints(cpuc, event);
+
+got_constraint:
 
-	return x86_get_event_constraints(cpuc, event);
+	/*
+	 * Must enabled before any actual event because any event may be
+	 * combined with LBR.
+	 */
+	if (intel_pmu_has_lbr(event))
+		intel_pmu_lbr_enable(event);
+
+	return c;
 }
 
 static void
@@ -1413,6 +1416,14 @@ intel_put_shared_regs_event_constraints(struct cpu_hw_events *cpuc,
 static void intel_put_event_constraints(struct cpu_hw_events *cpuc,
 					struct perf_event *event)
 {
+	/*
+	 * Must disabled after any actual event because any event may be
+	 * combined with LBR, but must be done before releasing the shared
+	 * regs resource since that protects the LBR state.
+	 */
+	if (intel_pmu_has_lbr(event))
+		intel_pmu_lbr_disable(event);
+
 	intel_put_shared_regs_event_constraints(cpuc, event);
 }
 
@@ -1623,8 +1634,7 @@ static __initconst const struct x86_pmu core_pmu = {
 	 * the generic event period:
 	 */
 	.max_period		= (1ULL << 31) - 1,
-	.get_event_constraints	= intel_get_event_constraints,
-	.put_event_constraints	= intel_put_event_constraints,
+	.get_event_constraints	= x86_get_event_constraints,
 	.event_constraints	= intel_core_event_constraints,
 	.guest_get_msrs		= core_guest_get_msrs,
 	.format_attrs		= intel_arch_formats_attr,