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authorJoel Stanley <joel@jms.id.au>2014-07-08 16:08:22 +0930
committerBenjamin Herrenschmidt <benh@kernel.crashing.org>2014-07-11 12:55:08 +1000
commitb50a6c584bb47b370f84bfd746770c0bbe7129b7 (patch)
parent4d9690dd56b0d18f2af8a9d4a279cb205aae3345 (diff)
powerpc/perf: Clear MMCR2 when enabling PMU
On POWER8 when switching to a KVM guest we set bits in MMCR2 to freeze the PMU counters. Aside from on boot they are then never reset, resulting in stuck perf counters for any user in the guest or host. We now set MMCR2 to 0 whenever enabling the PMU, which provides a sane state for perf to use the PMU counters under either the guest or the host. This was manifesting as a bug with ppc64_cpu --frequency: $ sudo ppc64_cpu --frequency WARNING: couldn't run on cpu 0 WARNING: couldn't run on cpu 8 ... WARNING: couldn't run on cpu 144 WARNING: couldn't run on cpu 152 min: 18446744073.710 GHz (cpu -1) max: 0.000 GHz (cpu -1) avg: 0.000 GHz The command uses a perf counter to measure CPU cycles over a fixed amount of time, in order to approximate the frequency of the machine. The counters were returning zero once a guest was started, regardless of weather it was still running or had been shut down. By dumping the value of MMCR2, it was observed that once a guest is running MMCR2 is set to 1s - which stops counters from running: $ sudo sh -c 'echo p > /proc/sysrq-trigger' CPU: 0 PMU registers, ppmu = POWER8 n_counters = 6 PMC1: 5b635e38 PMC2: 00000000 PMC3: 00000000 PMC4: 00000000 PMC5: 1bf5a646 PMC6: 5793d378 PMC7: deadbeef PMC8: deadbeef MMCR0: 0000000080000000 MMCR1: 000000001e000000 MMCRA: 0000040000000000 MMCR2: fffffffffffffc00 EBBHR: 0000000000000000 EBBRR: 0000000000000000 BESCR: 0000000000000000 SIAR: 00000000000a51cc SDAR: c00000000fc40000 SIER: 0000000001000000 This is done unconditionally in book3s_hv_interrupts.S upon entering the guest, and the original value is only save/restored if the host has indicated it was using the PMU. This is okay, however the user of the PMU needs to ensure that it is in a defined state when it starts using it. Fixes: e05b9b9e5c10 ("powerpc/perf: Power8 PMU support") Cc: stable@vger.kernel.org Signed-off-by: Joel Stanley <joel@jms.id.au> Acked-by: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
1 files changed, 3 insertions, 0 deletions
diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index a2ff1bda706a..bae697cd5925 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -1300,6 +1300,9 @@ static void power_pmu_enable(struct pmu *pmu)
write_mmcr0(cpuhw, mmcr0);
+ if (ppmu->flags & PPMU_ARCH_207S)
+ mtspr(SPRN_MMCR2, 0);
* Enable instruction sampling if necessary

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