Re: [PATCH v8 3/9] perf/amd/iommu: Misc fix up perf_iommu_read

Peter Zijlstra
2017-01-23T12:33:38Z

On Mon, Jan 16, 2017 at 01:23:30AM -0600, Suravee Suthikulpanit wrote:
> static void perf_iommu_read(struct perf_event *event)
> {
> - u64 count = 0ULL;
> - u64 prev_raw_count = 0ULL;
> - u64 delta = 0ULL;
> + u64 count, prev;
> + s64 delta;

I did send that email where I told I was mistaken with that suggestion,
right? Since the counter always increments (it does, right), a negative
delta does not make sense.

> struct hw_perf_event *hwc = &event->hw;
>
> amd_iommu_pc_get_set_reg_val(_GET_DEVID(event),
> @@ -330,18 +329,20 @@ static void perf_iommu_read(struct perf_event *event)
> IOMMU_PC_COUNTER_REG, &count, false);
>
> /* IOMMU pc counter register is only 48 bits */
> - count &= 0xFFFFFFFFFFFFULL;
> + count &= GENMASK_ULL(48, 0);

Why do you need that at all? If the counter is only 48 bits, what does
the hardware do with the upper bits?

>
> - 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);

I'm still not convinced you can do away with that cmpxchg.

>
> - /* 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.
> + */
> + local64_set(&hwc->prev_count, count);
> +
> + /* Handle 48-bit counter overflow */
> + delta = (count << 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
>