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

[Wei Huang]

On 11/14/2016 09:12 AM, Christopher Covington wrote:
> 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]
> 

Thanks. I did some research based on your info and came back with the
following proposals (Cov, correct me if I am wrong):

By comparing A57 TRM (page 394 in [1]) with A15 TRM (page 273 in [2]), I
think this 64-bit cycle register is only available when running under
aarch32 compatibility mode on ARMv8 because it is not specified in A15
TRM. To further verify it, I tested 32-bit pmu code on QEMU with TCG
mode. The result is: accessing 64-bit PMCCNTR using the following
assembly failed on A15:

   volatile("mrrc p15, 0, %0, %1, c9" : "=r" (lo), "=r" (hi));
or
   volatile("mrrc p15, 0, %Q0, %R0, c9" : "=r" (val));

Given this difference, I think there are two solutions for 64-bit
AArch32 pmccntr_read, as requested by Drew:

1) The PMU unit testing code tells if it is running under ARMv7 or under
AArch32-compability mode. When it is running ARMv7, such as A15, let us
use "MRC p15,0,<Rt>,c9,c13,0" and clear the upper 32-bit as 0. Otherwise
use "MRRC p15,0,<Rt>,<Rt2>,c9".

2) Returns 64-bit results for ARM pmccntr_read(). But we only uses "MRC
p15,0,<Rt>,c9,c13,0" and always clear the upper 32-bit as 0. This will
be the same as the original code.

Thoughts?

-Wei

[1] A57 TRM,
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0488c/DDI0488C_cortex_a57_mpcore_r1p0_trm.pdf
[2] A15 TRM,
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0438c/DDI0438C_cortex_a15_r2p0_trm.pdf

> Regards,
> Cov
>