Re: [kvm-unit-tests PATCH v10 3/3] arm: pmu: Add CPI checking

[Wei Huang]

On 11/21/2016 03:40 PM, Christopher Covington wrote:
> Hi Wei,
> 
> On 11/21/2016 03:24 PM, Wei Huang wrote:
>> From: Christopher Covington <cov@xxxxxxxxxxxxxx>
> 
> I really appreciate your work on these patches. If for any or all of these
> you have more lines added/modified than me (or using any other better
> metric), please make sure to change the author to be you with
> `git commit --amend --reset-author` or equivalent.

Sure, I will if needed. Regarding your comments below, I will fix the
patch series after Drew's comments, if any.

> 
>> Calculate the numbers of cycles per instruction (CPI) implied by ARM
>> PMU cycle counter values. The code includes a strict checking facility
>> intended for the -icount option in TCG mode in the configuration file.
>>
>> Signed-off-by: Christopher Covington <cov@xxxxxxxxxxxxxx>
>> Signed-off-by: Wei Huang <wei@xxxxxxxxxx>
>> ---
>>  arm/pmu.c         | 119 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>  arm/unittests.cfg |  14 +++++++
>>  2 files changed, 132 insertions(+), 1 deletion(-)
>>
>> diff --git a/arm/pmu.c b/arm/pmu.c
>> index 176b070..129ef1e 100644
>> --- a/arm/pmu.c
>> +++ b/arm/pmu.c
>> @@ -104,6 +104,25 @@ static inline uint32_t id_dfr0_read(void)
>>  	asm volatile("mrc p15, 0, %0, c0, c1, 2" : "=r" (val));
>>  	return val;
>>  }
>> +
>> +/*
>> + * Extra instructions inserted by the compiler would be difficult to compensate
>> + * for, so hand assemble everything between, and including, the PMCR accesses
>> + * to start and stop counting. Total cycles = isb + mcr + 2*loop = 2 + 2*loop.
                                   ^^^^^^^^^^^^
I will change the comment above to "Total instrs".

>> + */
>> +static inline void precise_cycles_loop(int loop, uint32_t pmcr)
> 
> Nit: I would call this precise_instrs_loop. How many cycles it takes is
> IMPLEMENTATION DEFINED.

You are right. The cycle indeed depends on the design. Will fix.

> 
>> +{
>> +	asm volatile(
>> +	"	mcr	p15, 0, %[pmcr], c9, c12, 0\n"
>> +	"	isb\n"
>> +	"1:	subs	%[loop], %[loop], #1\n"
>> +	"	bgt	1b\n"
> 
> Is there any chance we might need an isb here, to prevent the stop from happening
> before or during the loop? Where ISBs are required, the Linux best practice is to

In theory, I think this can happen when mcr is executed before all loop
instructions completed, causing pmccntr_read() to miss some cycles. But
QEMU TCG mode doesn't support out-order-execution. So the test
condition, "cpi > 0 && cycles != i * cpi", will never be TRUE. Because
cpi==0 in KVM, this same test condition won't be TRUE under KVM mode either.

> diligently comment why they are needed. Perhaps it would be a good habit to
> carry over into kvm-unit-tests.

Agreed. Most isb() instructions were added following CP15 writes (not
all CP15 writes, but at limited locations). We tried to follow what
Linux kernel does in perf_event.c. If you feel that any isb() place
needs special comment, I will be more than happy to add it.

<snip>