Re: [PATCH v7 1/7] perf/amd/iommu: Misc fix up perf_iommu_read

[Peter Zijlstra]

On Sun, Jan 15, 2017 at 09:36:10AM +0700, Suravee Suthikulpanit wrote:
> Peter,
> 
> On 1/11/17 18:57, Peter Zijlstra wrote:
> >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.
> 
> I'm changing the u64 to s64 ..... (see below)
> 
> >
> >>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;
> 
> .... (here) because we had a discussion (https://lkml.org/lkml/2016/2/18/325),
> and you suggested the following:
> 
>     Your overflow handling is broken, you want delta to be s64. Otherwise:
> 
> 	delta >>= COUNTER_SHIFT;
> 
>     ends up as a SHR and you loose the MSB sign bits.

Yah, I was wrong :-), please see:

  7f612a7f0bc1 ("perf/x86: Fix full width counter, counter overflow")


> >> 	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?
> 
> Not sure if I understand you point here. When you say sample, I assume you mean
> the profiling mode used w/ perf record. These counters are not supported for
> sampling mode. So, we only perf stat (i.e. counting mode).

But you can group them with a software event, right? And then that
software event can do sampling (using a timer for instance) and read the
other counters in the group using PERF_SAMPLE_READ and
PERF_FORMAT_GROUP.