Re: [Qemu-devel] [kvm-unit-tests PATCHv4 3/3] arm: pmu: Add CPI checking

[Christopher Covington]

Hi Drew,

I appreciate your feedback on these patches.

On 10/18/2015 02:28 PM, Andrew Jones wrote:

>> --- a/arm/pmu.c
>> +++ b/arm/pmu.c
>> @@ -37,6 +37,18 @@ static inline unsigned long get_pmccntr(void)
>>      asm volatile("mrc p15, 0, %0, c9, c13, 0" : "=r" (cycles));
>>      return cycles;
>>  }
>> +
>> +static inline void loop(int i, uint32_t pmcr)
>> +{
>> +    uint32_t z = 0;
>> +
>> +    asm volatile(
>> +            "       mcr p15, 0, %[pmcr], c9, c12, 0\n"
>> +            "       1: subs %[i], %[i], #1\n"
>> +            "       bgt 1b\n"
>> +            "       mcr p15, 0, %[z], c9, c12, 0\n"
>> +    : [i] "+r" (i) : [pmcr] "r" (pmcr), [z] "r" (z) : "cc");
> 
> Assembly is always ugly, but we can do a bit better formatting with tabs
> 
>       asm volatile(
>       "       mcr     p15, 0, %[pmcr], c9, c12, 0\n"
>       "1:     subs    %[i], %[i], #1\n"
>       "       bgt     1b\n"
>       "       mcr     p15, 0, %[z], c9, c12, 0\n"
>       : [i] "+r" (i)
>       : [pmcr] "r" (pmcr), [z] "r" (z)
>       : "cc");
> 
> Actually it can be even cleaner because you already created set_pmcr()
> 
>       set_pmcr(pmcr);
> 
>       asm volatile(
>       "1:     subs    %0, %0, #1\n"
>       "       bgt     1b\n"
>       : "+r" (i) : : "cc");
> 
>       set_pmcr(0);

Is there any way to ensure that the compiler won't for example put a `mov rd,
#0` between the `bgt 1b` and the `mcr <pmcr>, rn`?

>> @@ -125,12 +147,79 @@ static bool check_cycles_increase(void)
>>      return true;
>>  }
>>  
>> -int main(void)
>> +/*
>> + * Execute a known number of guest instructions. Only odd instruction counts
>> + * greater than or equal to 3 are supported by the in-line assembly code. 
>> The
> 
> Not all odd counts, right? But rather all multiples of 3? IIUC this is because
> the loop is two instructions (sub + branch), and then the clearing of the pmcr
> register counts as the 3rd?

Clearing the PMCR doesn't happen as part of the loop, but as part of the loop
exit or epilogue.

total_instrs = iteration_count * loop_instrs + eipilogue_instrs
total_instrs = iteration_count * 2 + 1

Thanks,
Christopher Covington

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