[PATCH] perf/intel: Remove Perfmon-v4 counter_freezing support
From: Peter Zijlstra
Date: Tue Nov 10 2020 - 10:37:50 EST
On Tue, Nov 10, 2020 at 04:12:57PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 09, 2020 at 10:12:37AM +0800, Like Xu wrote:
> > The Precise Event Based Sampling(PEBS) supported on Intel Ice Lake server
> > platforms can provide an architectural state of the instruction executed
> > after the instruction that caused the event. This patch set enables the
> > the PEBS via DS feature for KVM (also non) Linux guest on the Ice Lake.
> > The Linux guest can use PEBS feature like native:
> >
> > # perf record -e instructions:ppp ./br_instr a
> > # perf record -c 100000 -e instructions:pp ./br_instr a
> >
> > If the counter_freezing is not enabled on the host, the guest PEBS will
> > be disabled on purpose when host is using PEBS facility. By default,
> > KVM disables the co-existence of guest PEBS and host PEBS.
>
> Uuhh, what?!? counter_freezing should never be enabled, its broken. Let
> me go delete all that code.
---
Subject: perf/intel: Remove Perfmon-v4 counter_freezing support
Perfmon-v4 counter freezing is fundamentally broken; remove this default
disabled code to make sure nobody uses it.
The feature is called Freeze-on-PMI in the SDM, and if it would do that,
there wouldn't actually be a problem, *however* it does something subtly
different. It globally disables the whole PMU when it raises the PMI,
not when the PMI hits.
This means there's a window between the PMI getting raised and the PMI
actually getting served where we loose events and this violates the
perf counter independence. That is, a counting event should not result
in a different event count when there is a sampling event co-scheduled.
This is known to break existing software.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/events/intel/core.c | 152 -------------------------------------------
arch/x86/events/perf_event.h | 3 +-
2 files changed, 1 insertion(+), 154 deletions(-)
diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index c79748f6921d..9909dfa6fb12 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2121,18 +2121,6 @@ static void intel_tfa_pmu_enable_all(int added)
intel_pmu_enable_all(added);
}
-static void enable_counter_freeze(void)
-{
- update_debugctlmsr(get_debugctlmsr() |
- DEBUGCTLMSR_FREEZE_PERFMON_ON_PMI);
-}
-
-static void disable_counter_freeze(void)
-{
- update_debugctlmsr(get_debugctlmsr() &
- ~DEBUGCTLMSR_FREEZE_PERFMON_ON_PMI);
-}
-
static inline u64 intel_pmu_get_status(void)
{
u64 status;
@@ -2696,95 +2684,6 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
return handled;
}
-static bool disable_counter_freezing = true;
-static int __init intel_perf_counter_freezing_setup(char *s)
-{
- bool res;
-
- if (kstrtobool(s, &res))
- return -EINVAL;
-
- disable_counter_freezing = !res;
- return 1;
-}
-__setup("perf_v4_pmi=", intel_perf_counter_freezing_setup);
-
-/*
- * Simplified handler for Arch Perfmon v4:
- * - We rely on counter freezing/unfreezing to enable/disable the PMU.
- * This is done automatically on PMU ack.
- * - Ack the PMU only after the APIC.
- */
-
-static int intel_pmu_handle_irq_v4(struct pt_regs *regs)
-{
- struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
- int handled = 0;
- bool bts = false;
- u64 status;
- int pmu_enabled = cpuc->enabled;
- int loops = 0;
-
- /* PMU has been disabled because of counter freezing */
- cpuc->enabled = 0;
- if (test_bit(INTEL_PMC_IDX_FIXED_BTS, cpuc->active_mask)) {
- bts = true;
- intel_bts_disable_local();
- handled = intel_pmu_drain_bts_buffer();
- handled += intel_bts_interrupt();
- }
- status = intel_pmu_get_status();
- if (!status)
- goto done;
-again:
- intel_pmu_lbr_read();
- if (++loops > 100) {
- static bool warned;
-
- if (!warned) {
- WARN(1, "perfevents: irq loop stuck!\n");
- perf_event_print_debug();
- warned = true;
- }
- intel_pmu_reset();
- goto done;
- }
-
-
- handled += handle_pmi_common(regs, status);
-done:
- /* Ack the PMI in the APIC */
- apic_write(APIC_LVTPC, APIC_DM_NMI);
-
- /*
- * The counters start counting immediately while ack the status.
- * Make it as close as possible to IRET. This avoids bogus
- * freezing on Skylake CPUs.
- */
- if (status) {
- intel_pmu_ack_status(status);
- } else {
- /*
- * CPU may issues two PMIs very close to each other.
- * When the PMI handler services the first one, the
- * GLOBAL_STATUS is already updated to reflect both.
- * When it IRETs, the second PMI is immediately
- * handled and it sees clear status. At the meantime,
- * there may be a third PMI, because the freezing bit
- * isn't set since the ack in first PMI handlers.
- * Double check if there is more work to be done.
- */
- status = intel_pmu_get_status();
- if (status)
- goto again;
- }
-
- if (bts)
- intel_bts_enable_local();
- cpuc->enabled = pmu_enabled;
- return handled;
-}
-
/*
* This handler is triggered by the local APIC, so the APIC IRQ handling
* rules apply:
@@ -4081,9 +3980,6 @@ static void intel_pmu_cpu_starting(int cpu)
if (x86_pmu.version > 1)
flip_smm_bit(&x86_pmu.attr_freeze_on_smi);
- if (x86_pmu.counter_freezing)
- enable_counter_freeze();
-
/* Disable perf metrics if any added CPU doesn't support it. */
if (x86_pmu.intel_cap.perf_metrics) {
union perf_capabilities perf_cap;
@@ -4154,9 +4050,6 @@ static void free_excl_cntrs(struct cpu_hw_events *cpuc)
static void intel_pmu_cpu_dying(int cpu)
{
fini_debug_store_on_cpu(cpu);
-
- if (x86_pmu.counter_freezing)
- disable_counter_freeze();
}
void intel_cpuc_finish(struct cpu_hw_events *cpuc)
@@ -4548,39 +4441,6 @@ static __init void intel_nehalem_quirk(void)
}
}
-static const struct x86_cpu_desc counter_freezing_ucodes[] = {
- INTEL_CPU_DESC(INTEL_FAM6_ATOM_GOLDMONT, 2, 0x0000000e),
- INTEL_CPU_DESC(INTEL_FAM6_ATOM_GOLDMONT, 9, 0x0000002e),
- INTEL_CPU_DESC(INTEL_FAM6_ATOM_GOLDMONT, 10, 0x00000008),
- INTEL_CPU_DESC(INTEL_FAM6_ATOM_GOLDMONT_D, 1, 0x00000028),
- INTEL_CPU_DESC(INTEL_FAM6_ATOM_GOLDMONT_PLUS, 1, 0x00000028),
- INTEL_CPU_DESC(INTEL_FAM6_ATOM_GOLDMONT_PLUS, 8, 0x00000006),
- {}
-};
-
-static bool intel_counter_freezing_broken(void)
-{
- return !x86_cpu_has_min_microcode_rev(counter_freezing_ucodes);
-}
-
-static __init void intel_counter_freezing_quirk(void)
-{
- /* Check if it's already disabled */
- if (disable_counter_freezing)
- return;
-
- /*
- * If the system starts with the wrong ucode, leave the
- * counter-freezing feature permanently disabled.
- */
- if (intel_counter_freezing_broken()) {
- pr_info("PMU counter freezing disabled due to CPU errata,"
- "please upgrade microcode\n");
- x86_pmu.counter_freezing = false;
- x86_pmu.handle_irq = intel_pmu_handle_irq;
- }
-}
-
/*
* enable software workaround for errata:
* SNB: BJ122
@@ -4966,9 +4826,6 @@ __init int intel_pmu_init(void)
max((int)edx.split.num_counters_fixed, assume);
}
- if (version >= 4)
- x86_pmu.counter_freezing = !disable_counter_freezing;
-
if (boot_cpu_has(X86_FEATURE_PDCM)) {
u64 capabilities;
@@ -5090,7 +4947,6 @@ __init int intel_pmu_init(void)
case INTEL_FAM6_ATOM_GOLDMONT:
case INTEL_FAM6_ATOM_GOLDMONT_D:
- x86_add_quirk(intel_counter_freezing_quirk);
memcpy(hw_cache_event_ids, glm_hw_cache_event_ids,
sizeof(hw_cache_event_ids));
memcpy(hw_cache_extra_regs, glm_hw_cache_extra_regs,
@@ -5117,7 +4973,6 @@ __init int intel_pmu_init(void)
break;
case INTEL_FAM6_ATOM_GOLDMONT_PLUS:
- x86_add_quirk(intel_counter_freezing_quirk);
memcpy(hw_cache_event_ids, glp_hw_cache_event_ids,
sizeof(hw_cache_event_ids));
memcpy(hw_cache_extra_regs, glp_hw_cache_extra_regs,
@@ -5577,13 +5432,6 @@ __init int intel_pmu_init(void)
pr_cont("full-width counters, ");
}
- /*
- * For arch perfmon 4 use counter freezing to avoid
- * several MSR accesses in the PMI.
- */
- if (x86_pmu.counter_freezing)
- x86_pmu.handle_irq = intel_pmu_handle_irq_v4;
-
if (x86_pmu.intel_cap.perf_metrics)
x86_pmu.intel_ctrl |= 1ULL << GLOBAL_CTRL_EN_PERF_METRICS;
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 10032f023fcc..4084fa31cc21 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -681,8 +681,7 @@ struct x86_pmu {
/* PMI handler bits */
unsigned int late_ack :1,
- enabled_ack :1,
- counter_freezing :1;
+ enabled_ack :1;
/*
* sysfs attrs
*/