* [PATCH 2/2] powerpc/perf: Add lost exception workaround
2014-02-28 6:29 [PATCH 1/2] powerpc: Add a cpu feature CPU_FTR_PMAO_BUG Michael Ellerman
@ 2014-02-28 6:29 ` Michael Ellerman
0 siblings, 0 replies; 2+ messages in thread
From: Michael Ellerman @ 2014-02-28 6:29 UTC (permalink / raw)
To: linuxppc-dev
Some power8 revisions have a hardware bug where we can lose a PMU
exception, this commit adds a workaround to detect the bad condition and
rectify the situation.
See the comment in the commit for a full description.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
arch/powerpc/include/asm/reg.h | 2 +
arch/powerpc/perf/core-book3s.c | 100 +++++++++++++++++++++++++++++++++++++++-
arch/powerpc/perf/power8-pmu.c | 5 ++
3 files changed, 105 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index 90c06ec..3003472 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -670,6 +670,7 @@
#define MMCR0_PMC1CE 0x00008000UL /* PMC1 count enable*/
#define MMCR0_PMCjCE 0x00004000UL /* PMCj count enable*/
#define MMCR0_TRIGGER 0x00002000UL /* TRIGGER enable */
+#define MMCR0_PMAO_SYNC 0x00000800UL /* PMU interrupt is synchronous */
#define MMCR0_PMAO 0x00000080UL /* performance monitor alert has occurred, set to 0 after handling exception */
#define MMCR0_SHRFC 0x00000040UL /* SHRre freeze conditions between threads */
#define MMCR0_FC56 0x00000010UL /* freeze counters 5 and 6 */
@@ -703,6 +704,7 @@
#define SPRN_EBBHR 804 /* Event based branch handler register */
#define SPRN_EBBRR 805 /* Event based branch return register */
#define SPRN_BESCR 806 /* Branch event status and control register */
+#define BESCR_GE 0x8000000000000000ULL /* Global Enable */
#define SPRN_WORT 895 /* Workload optimization register - thread */
#define SPRN_PMC1 787
diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 67cf220..72cdd05 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -120,6 +120,7 @@ static inline void power_pmu_bhrb_enable(struct perf_event *event) {}
static inline void power_pmu_bhrb_disable(struct perf_event *event) {}
void power_pmu_flush_branch_stack(void) {}
static inline void power_pmu_bhrb_read(struct cpu_hw_events *cpuhw) {}
+static void pmao_restore_workaround(bool ebb) { }
#endif /* CONFIG_PPC32 */
static bool regs_use_siar(struct pt_regs *regs)
@@ -545,10 +546,18 @@ static unsigned long ebb_switch_in(bool ebb, unsigned long mmcr0)
/* Enable EBB and read/write to all 6 PMCs for userspace */
mmcr0 |= MMCR0_EBE | MMCR0_PMCC_U6;
- /* Add any bits from the user reg, FC or PMAO */
+ /*
+ * Add any bits from the user MMCR0, FC or PMAO. This is compatible
+ * with pmao_restore_workaround() because we may add PMAO but we never
+ * clear it here.
+ */
mmcr0 |= current->thread.mmcr0;
- /* Be careful not to set PMXE if userspace had it cleared */
+ /*
+ * Be careful not to set PMXE if userspace had it cleared. This is also
+ * compatible with pmao_restore_workaround() because it has already
+ * cleared PMXE and we leave PMAO alone.
+ */
if (!(current->thread.mmcr0 & MMCR0_PMXE))
mmcr0 &= ~MMCR0_PMXE;
@@ -559,6 +568,91 @@ static unsigned long ebb_switch_in(bool ebb, unsigned long mmcr0)
out:
return mmcr0;
}
+
+static void pmao_restore_workaround(bool ebb)
+{
+ unsigned pmcs[6];
+
+ if (!cpu_has_feature(CPU_FTR_PMAO_BUG))
+ return;
+
+ /*
+ * On POWER8 there is a hardware defect which affects the PMU context
+ * switch logic, ie. power_pmu_disable/enable().
+ *
+ * When a counter overflows PMXE is cleared and FC/PMAO is set in MMCR0
+ * by the hardware. Sometime later the actual PMU exception is
+ * delivered.
+ *
+ * If we context switch, or simply disable/enable, the PMU prior to the
+ * exception arriving, the exception will be lost when we clear PMAO.
+ *
+ * When we reenable the PMU, we will write the saved MMCR0 with PMAO
+ * set, and this _should_ generate an exception. However because of the
+ * defect no exception is generated when we write PMAO, and we get
+ * stuck with no counters counting but no exception delivered.
+ *
+ * The workaround is to detect this case and tweak the hardware to
+ * create another pending PMU exception.
+ *
+ * We do that by setting up PMC6 (cycles) for an imminent overflow and
+ * enabling the PMU. That causes a new exception to be generated in the
+ * chip, but we don't take it yet because we have interrupts hard
+ * disabled. We then write back the PMU state as we want it to be seen
+ * by the exception handler. When we reenable interrupts the exception
+ * handler will be called and see the correct state.
+ *
+ * The logic is the same for EBB, except that the exception is gated by
+ * us having interrupts hard disabled as well as the fact that we are
+ * not in userspace. The exception is finally delivered when we return
+ * to userspace.
+ */
+
+ /* Only if PMAO is set and PMAO_SYNC is clear */
+ if ((current->thread.mmcr0 & (MMCR0_PMAO | MMCR0_PMAO_SYNC)) != MMCR0_PMAO)
+ return;
+
+ /* If we're doing EBB, only if BESCR[GE] is set */
+ if (ebb && !(current->thread.bescr & BESCR_GE))
+ return;
+
+ /*
+ * We are already soft-disabled in power_pmu_enable(). We need to hard
+ * enable to actually prevent the PMU exception from firing.
+ */
+ hard_irq_disable();
+
+ /*
+ * This is a bit gross, but we know we're on POWER8 and have 6 PMCs.
+ * Using read/write_pmc() in a for loop adds 12 function calls and
+ * almost doubles our code size.
+ */
+ pmcs[0] = mfspr(SPRN_PMC1);
+ pmcs[1] = mfspr(SPRN_PMC2);
+ pmcs[2] = mfspr(SPRN_PMC3);
+ pmcs[3] = mfspr(SPRN_PMC4);
+ pmcs[4] = mfspr(SPRN_PMC5);
+ pmcs[5] = mfspr(SPRN_PMC6);
+
+ /* Ensure all freeze bits are unset */
+ mtspr(SPRN_MMCR2, 0);
+
+ /* Set up PMC6 to overflow in one cycle */
+ mtspr(SPRN_PMC6, 0x7FFFFFFE);
+
+ /* Enable exceptions and unfreeze PMC6 */
+ mtspr(SPRN_MMCR0, MMCR0_PMXE | MMCR0_PMCjCE | MMCR0_PMAO);
+
+ /* Now we need to refreeze and restore the PMCs */
+ mtspr(SPRN_MMCR0, MMCR0_FC | MMCR0_PMAO);
+
+ mtspr(SPRN_PMC1, pmcs[0]);
+ mtspr(SPRN_PMC2, pmcs[1]);
+ mtspr(SPRN_PMC3, pmcs[2]);
+ mtspr(SPRN_PMC4, pmcs[3]);
+ mtspr(SPRN_PMC5, pmcs[4]);
+ mtspr(SPRN_PMC6, pmcs[5]);
+}
#endif /* CONFIG_PPC64 */
static void perf_event_interrupt(struct pt_regs *regs);
@@ -1144,6 +1238,8 @@ static void power_pmu_enable(struct pmu *pmu)
cpuhw->mmcr[0] |= MMCR0_PMXE | MMCR0_FCECE;
out_enable:
+ pmao_restore_workaround(ebb);
+
mmcr0 = ebb_switch_in(ebb, cpuhw->mmcr[0]);
mb();
diff --git a/arch/powerpc/perf/power8-pmu.c b/arch/powerpc/perf/power8-pmu.c
index 96cee20..64f04cf 100644
--- a/arch/powerpc/perf/power8-pmu.c
+++ b/arch/powerpc/perf/power8-pmu.c
@@ -10,6 +10,8 @@
* 2 of the License, or (at your option) any later version.
*/
+#define pr_fmt(fmt) "power8-pmu: " fmt
+
#include <linux/kernel.h>
#include <linux/perf_event.h>
#include <asm/firmware.h>
@@ -774,6 +776,9 @@ static int __init init_power8_pmu(void)
/* Tell userspace that EBB is supported */
cur_cpu_spec->cpu_user_features2 |= PPC_FEATURE2_EBB;
+ if (cpu_has_feature(CPU_FTR_PMAO_BUG))
+ pr_info("PMAO restore workaround active.\n");
+
return 0;
}
early_initcall(init_power8_pmu);
--
1.8.3.2
^ permalink raw reply related [flat|nested] 2+ messages in thread