[PATCH v2 13/54] perf: core/x86: Forbid PMI handler when guest own PMU

Mingwei Zhang
2024-05-06T05:30:51Z

If a guest PMI is delivered after VM-exit, the KVM maskable interrupt will
be held pending until EFLAGS.IF is set. In the meantime, if the logical
processor receives an NMI for any reason at all, perf_event_nmi_handler()
will be invoked. If there is any active perf event anywhere on the system,
x86_pmu_handle_irq() will be invoked, and it will clear
IA32_PERF_GLOBAL_STATUS. By the time KVM's PMI handler is invoked, it will
be a mystery which counter(s) overflowed.

When LVTPC is using KVM PMI vecotr, PMU is owned by guest, Host NMI let
x86_pmu_handle_irq() run, x86_pmu_handle_irq() restore PMU vector to NMI
and clear IA32_PERF_GLOBAL_STATUS, this breaks guest vPMU passthrough
environment.

So modify perf_event_nmi_handler() to check perf_guest_context_loaded,
and if so, to simply return without calling x86_pmu_handle_irq().

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
---
arch/x86/events/core.c | 19 ++++++++++++++++++-
include/linux/perf_event.h | 5 +++++
kernel/events/core.c | 5 +++++
3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 8167f2230d3a..c0f6e294fcad 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -726,7 +726,7 @@ EXPORT_SYMBOL_GPL(x86_perf_guest_exit);
* It will not be re-enabled in the NMI handler again, because enabled=0. After
* handling the NMI, disable_all will be called, which will not change the
* state either. If PMI hits after disable_all, the PMU is already disabled
- * before entering NMI handler. The NMI handler will not change the state
+ * before entering NMI handler. The NMI handler will no change the state
* either.
*
* So either situation is harmless.
@@ -1749,6 +1749,23 @@ perf_event_nmi_handler(unsigned int cmd, struct pt_regs *regs)
u64 finish_clock;
int ret;

+ /*
+ * When guest pmu context is loaded this handler should be forbidden from
+ * running, the reasons are:
+ * 1. After x86_perf_guest_enter() is called, and before cpu enter into
+ * non-root mode, NMI could happen, but x86_pmu_handle_irq() restore PMU
+ * to use NMI vector, which destroy KVM PMI vector setting.
+ * 2. When VM is running, host NMI other than PMI causes VM exit, KVM will
+ * call host NMI handler (vmx_vcpu_enter_exit()) first before KVM save
+ * guest PMU context (kvm_pmu_save_pmu_context()), as x86_pmu_handle_irq()
+ * clear global_status MSR which has guest status now, then this destroy
+ * guest PMU status.
+ * 3. After VM exit, but before KVM save guest PMU context, host NMI other
+ * than PMI could happen, x86_pmu_handle_irq() clear global_status MSR
+ * which has guest status now, then this destroy guest PMU status.
+ */
+ if (perf_is_guest_context_loaded())
+ return 0;
/*
* All PMUs/events that share this PMI handler should make sure to
* increment active_events for their events.
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index acf16676401a..5da7de42954e 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1736,6 +1736,7 @@ extern int perf_get_mediated_pmu(void);
extern void perf_put_mediated_pmu(void);
void perf_guest_enter(void);
void perf_guest_exit(void);
+bool perf_is_guest_context_loaded(void);
#else /* !CONFIG_PERF_EVENTS: */
static inline void *
perf_aux_output_begin(struct perf_output_handle *handle,
@@ -1830,6 +1831,10 @@ static inline int perf_get_mediated_pmu(void)
static inline void perf_put_mediated_pmu(void) { }
static inline void perf_guest_enter(void) { }
static inline void perf_guest_exit(void) { }
+static inline bool perf_is_guest_context_loaded(void)
+{
+ return false;
+}
#endif

#if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4c6daf5cc923..184d06c23391 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5895,6 +5895,11 @@ void perf_guest_exit(void)
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}

+bool perf_is_guest_context_loaded(void)
+{
+ return __this_cpu_read(perf_in_guest);
+}
+
/*
* Holding the top-level event's child_mutex means that any
* descendant process that has inherited this event will block
--
2.45.0.rc1.225.g2a3ae87e7f-goog