Re: [Qemu-devel] [kvm-unit-tests PATCHv5 2/3] arm: pmu: Check cycle count increases

[Andrew Jones <drjones@redhat.com>]

On Mon, Oct 26, 2015 at 11:38:49AM -0400, Christopher Covington wrote:
> Ensure that reads of the PMCCNTR_EL0 are monotonically increasing,
> even for the smallest delta of two subsequent reads.
> 
> Signed-off-by: Christopher Covington <cov@codeaurora.org>
> ---
>  arm/pmu.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
> 
> diff --git a/arm/pmu.c b/arm/pmu.c
> index 42d0ee1..c44d708 100644
> --- a/arm/pmu.c
> +++ b/arm/pmu.c
> @@ -14,6 +14,8 @@
>   */
>  #include "libcflat.h"
>  
> +#define NR_SAMPLES 10
> +
>  #if defined(__arm__)
>  static inline uint32_t get_pmcr(void)
>  {
> @@ -22,6 +24,25 @@ static inline uint32_t get_pmcr(void)
>  	asm volatile("mrc p15, 0, %0, c9, c12, 0" : "=r" (ret));
>  	return ret;
>  }
> +
> +static inline void set_pmcr(uint32_t pmcr)
> +{
> +	asm volatile("mcr p15, 0, %0, c9, c12, 0" : : "r" (pmcr));
> +}
> +
> +/*
> + * While PMCCNTR can be accessed as a 64 bit coprocessor register, returning 64
> + * bits doesn't seem worth the trouble when differential usage of the result is
> + * expected (with differences that can easily fit in 32 bits). So just return
> + * the lower 32 bits of the cycle count in AArch32.
> + */
> +static inline unsigned long get_pmccntr(void)
> +{
> +	unsigned long cycles;
> +
> +	asm volatile("mrc p15, 0, %0, c9, c13, 0" : "=r" (cycles));
> +	return cycles;
> +}
>  #elif defined(__aarch64__)
>  static inline uint32_t get_pmcr(void)
>  {
> @@ -30,6 +51,19 @@ static inline uint32_t get_pmcr(void)
>  	asm volatile("mrs %0, pmcr_el0" : "=r" (ret));
>  	return ret;
>  }
> +
> +static inline void set_pmcr(uint32_t pmcr)
> +{
> +	asm volatile("msr pmcr_el0, %0" : : "r" (pmcr));
> +}
> +
> +static inline unsigned long get_pmccntr(void)
> +{
> +	unsigned long cycles;
> +
> +	asm volatile("mrs %0, pmccntr_el0" : "=r" (cycles));
> +	return cycles;
> +}
>  #endif
>  
>  struct pmu_data {
> @@ -72,11 +106,37 @@ static bool check_pmcr(void)
>  	return pmu.implementer != 0;
>  }
>  
> +/*
> + * Ensure that the cycle counter progresses between back-to-back reads.
> + */
> +static bool check_cycles_increase(void)
> +{
> +	struct pmu_data pmu = { {0} };

One set of {} is enough, and looks better.

> +
> +	pmu.enable = 1;
> +	set_pmcr(pmu.pmcr_el0);
> +
> +	for (int i = 0; i < NR_SAMPLES; i++) {
> +		unsigned long a, b;
> +
> +		a = get_pmccntr();
> +		b = get_pmccntr();
> +
> +		if (a >= b) {
> +			printf("Read %ld then %ld.\n", a, b);
> +			return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
>  int main(void)
>  {
>  	report_prefix_push("pmu");
>  
>  	report("Control register", check_pmcr());
> +	report("Monotonically increasing cycle count", check_cycles_increase());
>  
>  	return report_summary();
>  }
> -- 
> Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
> 
>

Otherwise
Reviewed-by: Andrew Jones <drjones@redhat.com>