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

[Christopher Covington]

Hi Drew, Wei,

On 11/14/2016 05:05 AM, Andrew Jones wrote:
> On Fri, Nov 11, 2016 at 01:55:49PM -0600, Wei Huang wrote:
>>
>>
>> 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.

AArch32 System Register Descriptions
Performance Monitors registers
PMCCNTR, Performance Monitors Cycle Count Register

To access the PMCCNTR when accessing as a 32-bit register:
MRC p15,0,<Rt>,c9,c13,0 ; Read PMCCNTR[31:0] into Rt
MCR p15,0,<Rt>,c9,c13,0 ; Write Rt to PMCCNTR[31:0]. PMCCNTR[63:32] are 
unchanged

To access the PMCCNTR when accessing as a 64-bit register:
MRRC p15,0,<Rt>,<Rt2>,c9 ; Read PMCCNTR[31:0] into Rt and PMCCNTR[63:32] into 
Rt2
MCRR p15,0,<Rt>,<Rt2>,c9 ; Write Rt to PMCCNTR[31:0] and Rt2 to PMCCNTR[63:32]

Regards,
Cov

-- 
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm
Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code
Aurora Forum, a Linux Foundation Collaborative Project.