Re: [Qemu-devel] [kvm-unit-tests PATCH v8 2/3] arm: pmu: Check cycle count increases

[Andrew Jones]

Just crossed my mind that we're missing isb's.

On Tue, Nov 08, 2016 at 12:17:14PM -0600, Wei Huang wrote:
> From: Christopher Covington <address@hidden>
> 
> Ensure that reads of the PMCCNTR_EL0 are monotonically increasing,
> even for the smallest delta of two subsequent reads.
> 
> Signed-off-by: Christopher Covington <address@hidden>
> Signed-off-by: Wei Huang <address@hidden>
> ---
>  arm/pmu.c | 98 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 98 insertions(+)
> 
> diff --git a/arm/pmu.c b/arm/pmu.c
> index 0b29088..d5e3ac3 100644
> --- a/arm/pmu.c
> +++ b/arm/pmu.c
> @@ -14,6 +14,7 @@
>   */
>  #include "libcflat.h"
>  
> +#define PMU_PMCR_E         (1 << 0)
>  #define PMU_PMCR_N_SHIFT   11
>  #define PMU_PMCR_N_MASK    0x1f
>  #define PMU_PMCR_ID_SHIFT  16
> @@ -21,6 +22,10 @@
>  #define PMU_PMCR_IMP_SHIFT 24
>  #define PMU_PMCR_IMP_MASK  0xff
>  
> +#define PMU_CYCLE_IDX      31
> +
> +#define NR_SAMPLES 10
> +
>  #if defined(__arm__)
>  static inline uint32_t pmcr_read(void)
>  {
> @@ -29,6 +34,47 @@ static inline uint32_t pmcr_read(void)
>       asm volatile("mrc p15, 0, %0, c9, c12, 0" : "=r" (ret));
>       return ret;
>  }
> +
> +static inline void pmcr_write(uint32_t value)
> +{
> +     asm volatile("mcr p15, 0, %0, c9, c12, 0" : : "r" (value));
> +}
> +
> +static inline void pmselr_write(uint32_t value)
> +{
> +     asm volatile("mcr p15, 0, %0, c9, c12, 5" : : "r" (value));

Probably want an isb here, users will call this and then immediately
another PMU reg write, like is done below

> +}
> +
> +static inline void pmxevtyper_write(uint32_t value)
> +{
> +     asm volatile("mcr p15, 0, %0, c9, c13, 1" : : "r" (value));
> +}
> +
> +/*
> + * 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.

Also, while we're discussing confirming upper bits are as expected, I
guess we should confirm no overflow too. We should clear the overflow
bit PMOVSCLR_EL0.C before we use the counter, and then check it at some
point to confirm it's as expected. I guess that could be separate test
cases though.

> + */
> +static inline uint32_t pmccntr_read(void)
> +{
> +     uint32_t cycles;
> +
> +     asm volatile("mrc p15, 0, %0, c9, c13, 0" : "=r" (cycles));
> +     return cycles;
> +}
> +
> +static inline void pmcntenset_write(uint32_t value)
> +{
> +     asm volatile("mcr p15, 0, %0, c9, c12, 1" : : "r" (value));
> +}
> +
> +/* PMCCFILTR is an obsolete name for PMXEVTYPER31 in ARMv7 */
> +static inline void pmccfiltr_write(uint32_t value)
> +{
> +     pmselr_write(PMU_CYCLE_IDX);
> +     pmxevtyper_write(value);
> +}
>  #elif defined(__aarch64__)
>  static inline uint32_t pmcr_read(void)
>  {
> @@ -37,6 +83,29 @@ static inline uint32_t pmcr_read(void)
>       asm volatile("mrs %0, pmcr_el0" : "=r" (ret));
>       return ret;
>  }
> +
> +static inline void pmcr_write(uint32_t value)
> +{
> +     asm volatile("msr pmcr_el0, %0" : : "r" (value));
> +}
> +
> +static inline uint32_t pmccntr_read(void)
> +{
> +     uint32_t cycles;
> +
> +     asm volatile("mrs %0, pmccntr_el0" : "=r" (cycles));
> +     return cycles;
> +}
> +
> +static inline void pmcntenset_write(uint32_t value)
> +{
> +     asm volatile("msr pmcntenset_el0, %0" : : "r" (value));
> +}
> +
> +static inline void pmccfiltr_write(uint32_t value)
> +{
> +     asm volatile("msr pmccfiltr_el0, %0" : : "r" (value));
> +}
>  #endif
>  
>  /*
> @@ -63,11 +132,40 @@ static bool check_pmcr(void)
>       return ((pmcr >> PMU_PMCR_IMP_SHIFT) & PMU_PMCR_IMP_MASK) != 0;
>  }
>  
> +/*
> + * Ensure that the cycle counter progresses between back-to-back reads.
> + */
> +static bool check_cycles_increase(void)
> +{
> +     pmcr_write(pmcr_read() | PMU_PMCR_E);

Need isb() here

> +
> +     for (int i = 0; i < NR_SAMPLES; i++) {
> +             unsigned long a, b;
> +
> +             a = pmccntr_read();
> +             b = pmccntr_read();
> +
> +             if (a >= b) {
> +                     printf("Read %ld then %ld.\n", a, b);
> +                     return false;
> +             }
> +     }
> +
> +     pmcr_write(pmcr_read() & ~PMU_PMCR_E);
> +

Need isb() here

> +     return true;
> +}
> +
>  int main(void)
>  {
>       report_prefix_push("pmu");
>  
> +     /* init for PMU event access, right now only care about cycle count */
> +     pmcntenset_write(1 << PMU_CYCLE_IDX);
> +     pmccfiltr_write(0); /* count cycles in EL0, EL1, but not EL2 */

Need isb() here

> +
>       report("Control register", check_pmcr());
> +     report("Monotonically increasing cycle count", check_cycles_increase());
>  
>       return report_summary();
>  }
> -- 
> 1.8.3.1
> 
>

Thanks,
drew