[v2,5/5] perf: Correct perf sampling with guest VMs

Commit Message

Colton Lewis Sept. 11, 2024, 10:24 p.m. UTC

Previously any PMU overflow interrupt that fired while a VCPU was
loaded was recorded as a guest event whether it truly was or not. This
resulted in nonsense perf recordings that did not honor
perf_event_attr.exclude_guest and recorded guest IPs where it should
have recorded host IPs.

Rework the sampling logic to only record guest samples for events with
exclude_guest clear. This way any host-only events with exclude_guest
set will never see unexpected guest samples. The behaviour of events
with exclude_guest clear is unchanged.

Signed-off-by: Colton Lewis <[e-mail]>
---
 arch/arm64/include/asm/perf_event.h |  4 ----
 arch/arm64/kernel/perf_callchain.c  | 28 ----------------------------
 arch/x86/events/core.c              | 15 ++++-----------
 include/linux/perf_event.h          | 21 +++++++++++++++++++--
 kernel/events/core.c                | 21 +++++++++++++++++----
 5 files changed, 40 insertions(+), 49 deletions(-)

Comments

Sean Christopherson Sept. 11, 2024, 10:56 p.m. UTC | #1

On Wed, Sep 11, 2024, Colton Lewis wrote:
> Previously any PMU overflow interrupt that fired while a VCPU was
> loaded was recorded as a guest event whether it truly was or not. This
> resulted in nonsense perf recordings that did not honor
> perf_event_attr.exclude_guest and recorded guest IPs where it should
> have recorded host IPs.
> 
> Rework the sampling logic to only record guest samples for events with
> exclude_guest clear. This way any host-only events with exclude_guest
> set will never see unexpected guest samples. The behaviour of events
> with exclude_guest clear is unchanged.

Nit, "with exclude_guest clear" is easy to misread as simply "with exclude_guest"
(I did so at least three times).  Maybe 

  The behavior of exclude_guest=0 events is unchanged.

or

  The behavior of events without exclude_guest is unchanged.

I think it's also worth explicitly calling out that events that are configured
to sample both host and guest may still be prone to misattributing a PMI that
arrived in the host as a guest event, depending on the KVM arch and/or vendor
behavior.

Patch

diff --git a/arch/arm64/include/asm/perf_event.h b/arch/arm64/include/asm/perf_event.h
index 31a5584ed423..ee45b4e77347 100644
--- a/arch/arm64/include/asm/perf_event.h
+++ b/arch/arm64/include/asm/perf_event.h
@@ -10,10 +10,6 @@ 
 #include <asm/ptrace.h>
 
 #ifdef CONFIG_PERF_EVENTS
-struct pt_regs;
-extern unsigned long perf_arch_instruction_pointer(struct pt_regs *regs);
-extern unsigned long perf_arch_misc_flags(struct pt_regs *regs);
-#define perf_arch_misc_flags(regs)	perf_misc_flags(regs)
 #define perf_arch_bpf_user_pt_regs(regs) &regs->user_regs
 #endif
 
diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
index 01a9d08fc009..9b7f26b128b5 100644
--- a/arch/arm64/kernel/perf_callchain.c
+++ b/arch/arm64/kernel/perf_callchain.c
@@ -38,31 +38,3 @@  void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
 
 	arch_stack_walk(callchain_trace, entry, current, regs);
 }
-
-unsigned long perf_arch_instruction_pointer(struct pt_regs *regs)
-{
-	if (perf_guest_state())
-		return perf_guest_get_ip();
-
-	return instruction_pointer(regs);
-}
-
-unsigned long perf_arch_misc_flags(struct pt_regs *regs)
-{
-	unsigned int guest_state = perf_guest_state();
-	int misc = 0;
-
-	if (guest_state) {
-		if (guest_state & PERF_GUEST_USER)
-			misc |= PERF_RECORD_MISC_GUEST_USER;
-		else
-			misc |= PERF_RECORD_MISC_GUEST_KERNEL;
-	} else {
-		if (user_mode(regs))
-			misc |= PERF_RECORD_MISC_USER;
-		else
-			misc |= PERF_RECORD_MISC_KERNEL;
-	}
-
-	return misc;
-}
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index d51e5d24802b..5f119b9516dc 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2942,9 +2942,6 @@  static unsigned long code_segment_base(struct pt_regs *regs)
 
 unsigned long perf_arch_instruction_pointer(struct pt_regs *regs)
 {
-	if (perf_guest_state())
-		return perf_guest_get_ip();
-
 	return regs->ip + code_segment_base(regs);
 }
 
@@ -2974,14 +2971,10 @@  unsigned long perf_arch_misc_flags(struct pt_regs *regs)
 	unsigned int guest_state = perf_guest_state();
 	unsigned long misc = common_misc_flags(regs);
 
-	if (guest_state) {
-		misc |= perf_arch_guest_misc_flags(regs);
-	} else {
-		if (user_mode(regs))
-			misc |= PERF_RECORD_MISC_USER;
-		else
-			misc |= PERF_RECORD_MISC_KERNEL;
-	}
+	if (user_mode(regs))
+		misc |= PERF_RECORD_MISC_USER;
+	else
+		misc |= PERF_RECORD_MISC_KERNEL;
 
 	return misc;
 }
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index d061e327ad54..968f3edd95e4 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1633,8 +1633,9 @@  extern void perf_tp_event(u16 event_type, u64 count, void *record,
 			  struct task_struct *task);
 extern void perf_bp_event(struct perf_event *event, void *data);
 
-extern unsigned long perf_misc_flags(struct pt_regs *regs);
-extern unsigned long perf_instruction_pointer(struct pt_regs *regs);
+extern unsigned long perf_misc_flags(struct perf_event *event, struct pt_regs *regs);
+extern unsigned long perf_instruction_pointer(struct perf_event *event,
+					      struct pt_regs *regs);
 
 #ifndef perf_arch_misc_flags
 # define perf_arch_misc_flags(regs) \
@@ -1645,6 +1646,22 @@  extern unsigned long perf_instruction_pointer(struct pt_regs *regs);
 # define perf_arch_bpf_user_pt_regs(regs) regs
 #endif
 
+#ifndef perf_arch_guest_misc_flags
+static inline unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
+{
+	unsigned long guest_state = perf_guest_state();
+
+	if (guest_state & PERF_GUEST_USER)
+		return PERF_RECORD_MISC_GUEST_USER;
+
+	if (guest_state & PERF_GUEST_ACTIVE)
+		return PERF_RECORD_MISC_GUEST_KERNEL;
+
+	return 0;
+}
+# define perf_arch_guest_misc_flags(regs)	perf_arch_guest_misc_flags(regs)
+#endif
+
 static inline bool has_branch_stack(struct perf_event *event)
 {
 	return event->attr.sample_type & PERF_SAMPLE_BRANCH_STACK;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index eeabbf791a8c..c5e57c024d9a 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6921,13 +6921,26 @@  void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
 #endif
 
-unsigned long perf_misc_flags(struct pt_regs *regs)
+static bool should_sample_guest(struct perf_event *event)
 {
+	return !event->attr.exclude_guest && perf_guest_state();
+}
+
+unsigned long perf_misc_flags(struct perf_event *event,
+			      struct pt_regs *regs)
+{
+	if (should_sample_guest(event))
+		return perf_arch_guest_misc_flags(regs);
+
 	return perf_arch_misc_flags(regs);
 }
 
-unsigned long perf_instruction_pointer(struct pt_regs *regs)
+unsigned long perf_instruction_pointer(struct perf_event *event,
+				       struct pt_regs *regs)
 {
+	if (should_sample_guest(event))
+		return perf_guest_get_ip();
+
 	return perf_arch_instruction_pointer(regs);
 }
 
@@ -7743,7 +7756,7 @@  void perf_prepare_sample(struct perf_sample_data *data,
 	__perf_event_header__init_id(data, event, filtered_sample_type);
 
 	if (filtered_sample_type & PERF_SAMPLE_IP) {
-		data->ip = perf_instruction_pointer(regs);
+		data->ip = perf_instruction_pointer(event, regs);
 		data->sample_flags |= PERF_SAMPLE_IP;
 	}
 
@@ -7907,7 +7920,7 @@  void perf_prepare_header(struct perf_event_header *header,
 {
 	header->type = PERF_RECORD_SAMPLE;
 	header->size = perf_sample_data_size(data, event);
-	header->misc = perf_misc_flags(regs);
+	header->misc = perf_misc_flags(event, regs);
 
 	/*
 	 * If you're adding more sample types here, you likely need to do