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

[Wei Huang]

On 11/11/2016 01:43 AM, Andrew Jones wrote:
> 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));
>> +}
>> +
>> +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.
> 
> Like I said in the last review, I'd rather we not do this. We should
> return the full value and then the test case should confirm the upper
> 32 bits are zero.
> 

Unless I miss something in ARM documentation, ARMv7 PMCCNTR is a 32-bit
register. We can force it to a more coarse-grained cycle counter with
PMCR.D bit=1 (see below). But it is still not a 64-bit register. ARMv8
PMCCNTR_EL0 is a 64-bit register.

"The PMCR.D bit configures whether PMCCNTR increments once every clock
cycle, or once every 64 clock cycles. "

So I think the comment above in the code is an overstatement, which
should be deleted or moved down to ARMv8 pmccntr_read() below.

>> + */
>> +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);
>> +
>> +    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);
>> +
>> +    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 */
>> +
>>      report("Control register", check_pmcr());
>> +    report("Monotonically increasing cycle count", check_cycles_increase());
>>  
>>      return report_summary();
>>  }
>> -- 
>> 1.8.3.1
> 
> Besides needing to use u64's for registers that return u64's, it
> looks good to me.
> 
> drew
>