Messages in this thread | | | Date | Wed, 11 Jan 2017 12:57:35 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v7 1/7] perf/amd/iommu: Misc fix up perf_iommu_read |
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On Mon, Jan 09, 2017 at 09:33:41PM -0600, Suravee Suthikulpanit wrote: > This patch contains the following minor fixup: > * Fixed overflow handling since u64 delta would lose the MSB sign bit.
Please explain.. afaict this actually introduces a bug.
> diff --git a/arch/x86/events/amd/iommu.c b/arch/x86/events/amd/iommu.c > index b28200d..f387baf 100644 > --- a/arch/x86/events/amd/iommu.c > +++ b/arch/x86/events/amd/iommu.c > @@ -319,29 +319,30 @@ static void perf_iommu_start(struct perf_event *event, int flags) > > static void perf_iommu_read(struct perf_event *event) > { > - u64 count = 0ULL; > - u64 prev_raw_count = 0ULL; > - u64 delta = 0ULL; > + u64 cnt, prev; > + s64 delta; > struct hw_perf_event *hwc = &event->hw; > pr_debug("perf: amd_iommu:perf_iommu_read\n"); > > amd_iommu_pc_get_set_reg_val(_GET_DEVID(event), > _GET_BANK(event), _GET_CNTR(event), > - IOMMU_PC_COUNTER_REG, &count, false); > + IOMMU_PC_COUNTER_REG, &cnt, false); > > /* IOMMU pc counter register is only 48 bits */ > - count &= 0xFFFFFFFFFFFFULL; > + cnt &= GENMASK_ULL(48, 0); > > - prev_raw_count = local64_read(&hwc->prev_count); > - if (local64_cmpxchg(&hwc->prev_count, prev_raw_count, > - count) != prev_raw_count) > - return; > + prev = local64_read(&hwc->prev_count); > > - /* Handling 48-bit counter overflowing */ > - delta = (count << COUNTER_SHIFT) - (prev_raw_count << COUNTER_SHIFT); > + /* > + * Since we do not enable counter overflow interrupts, > + * we do not have to worry about prev_count changing on us. > + */
So you cannot group this event with a software event that reads this from their sample?
> + local64_set(&hwc->prev_count, cnt); > + > + /* Handle 48-bit counter overflow */ > + delta = (cnt << COUNTER_SHIFT) - (prev << COUNTER_SHIFT); > delta >>= COUNTER_SHIFT; > local64_add(delta, &event->count); > - > } > > static void perf_iommu_stop(struct perf_event *event, int flags) > -- > 1.8.3.1 >
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