[PATCH 1/2] powerpc: Add a cpu feature CPU_FTR_PMAO_BUG

[PATCH 1/2] powerpc: Add a cpu feature CPU_FTR_PMAO_BUG

[Michael Ellerman]

Some power8 revisions have a hardware bug where we can lose a
Performance Monitor (PMU) exception under certain circumstances.

We will be adding a workaround for this case, see the next commit for
details. The observed behaviour is that writing PMAO doesn't cause an
exception as we would expect, hence the name of the feature.

This commit just adds a CPU feature bit. We set it on all power8 cpus,
and then clear it at cpu setup time if we are on a revision where the
bug is fixed.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/include/asm/cputable.h   |  3 ++-
 arch/powerpc/kernel/cpu_setup_power.S | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
index 617cc76..56d3166 100644
--- a/arch/powerpc/include/asm/cputable.h
+++ b/arch/powerpc/include/asm/cputable.h
@@ -189,6 +189,7 @@ extern const char *powerpc_base_platform;
 #define	CPU_FTR_HAS_PPR			LONG_ASM_CONST(0x0200000000000000)
 #define CPU_FTR_DAWR			LONG_ASM_CONST(0x0400000000000000)
 #define CPU_FTR_DABRX			LONG_ASM_CONST(0x0800000000000000)
+#define CPU_FTR_PMAO_BUG		LONG_ASM_CONST(0x1000000000000000)
 
 #ifndef __ASSEMBLY__
 
@@ -444,7 +445,7 @@ extern const char *powerpc_base_platform;
 	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
 	    CPU_FTR_ICSWX | CPU_FTR_CFAR | CPU_FTR_HVMODE | CPU_FTR_VMX_COPY | \
 	    CPU_FTR_DBELL | CPU_FTR_HAS_PPR | CPU_FTR_DAWR | \
-	    CPU_FTR_ARCH_207S | CPU_FTR_TM_COMP)
+	    CPU_FTR_ARCH_207S | CPU_FTR_TM_COMP | CPU_FTR_PMAO_BUG)
 #define CPU_FTRS_CELL	(CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
 	    CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
 	    CPU_FTR_ALTIVEC_COMP | CPU_FTR_MMCRA | CPU_FTR_SMT | \
diff --git a/arch/powerpc/kernel/cpu_setup_power.S b/arch/powerpc/kernel/cpu_setup_power.S
index 37d1bb0..cb2da74 100644
--- a/arch/powerpc/kernel/cpu_setup_power.S
+++ b/arch/powerpc/kernel/cpu_setup_power.S
@@ -179,4 +179,19 @@ __init_PMU:
 	mtspr	SPRN_MMCR0,r5
 	mtspr	SPRN_MMCR1,r5
 	mtspr	SPRN_MMCR2,r5
+
+	/* Compare our PVR to POWER8 DD2 (0x4d0200) */
+	mfpvr	r5
+	lis	r6,0x4d
+	ori	r6,r6,0x0200
+	cmplw	r5,r6
+
+	/* If we are older then return */
+	bltlr
+
+	/* Otherwise clear CPU_FTR_PMAO_BUG */
+	ld	r5,CPU_SPEC_FEATURES(r4)
+	LOAD_REG_IMMEDIATE(r6,CPU_FTR_PMAO_BUG)
+	xor	r5,r5,r6
+	std	r5,CPU_SPEC_FEATURES(r4)
 	blr
-- 
1.8.3.2

[PATCH 2/2] powerpc/perf: Add lost exception workaround

[Michael Ellerman]

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