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

[Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

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
>