Re: [Qemu-devel] [PATCH] target-arm: Use common CPU cycle infrastructure

[Christopher Covington]

On 09/28/2015 06:05 PM, Alistair Francis wrote:
> On Thu, Sep 24, 2015 at 12:43 PM, Christopher Covington
> <address@hidden> wrote:
>> cpu_get_ticks() provides a common interface across targets for
>> calculating CPU cycles. Using this fixes PMCCNTR reads when -icount
>> is specified (previously a non-increasing value was returned).
>>
>> Signed-off-by: Christopher Covington <address@hidden>
>> ---
>>  target-arm/helper.c | 9 +++------
>>  1 file changed, 3 insertions(+), 6 deletions(-)
>>
>> diff --git a/target-arm/helper.c b/target-arm/helper.c
>> index 7dc49cb..32923fb 100644
>> --- a/target-arm/helper.c
>> +++ b/target-arm/helper.c
>> @@ -729,8 +729,7 @@ void pmccntr_sync(CPUARMState *env)
>>  {
>>      uint64_t temp_ticks;
>>
>> -    temp_ticks = muldiv64(qemu_clock_get_us(QEMU_CLOCK_VIRTUAL),
>> -                          get_ticks_per_sec(), 1000000);
>> +    temp_ticks = cpu_get_ticks();

> Also I don't think this is correct. cpu_get_ticks() returns the host
> CPU cycle counter, when in this case we want the guest cycles.

I too find the use of host CPU cycles quite perplexing. Paolo suggested it
[1]. Maybe there are timeouts in some software that tend to work better in
such a mode. Perhaps it is faster, although my intuition is that it's just
providing a facade of frequency scaling to the guest.

1. https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg00162.html

I like to declare guest instructions per guest CPU cycles = 1. As I understand
it, an "-icount 0" pair of parameters is how to do this in QEMU for x86. I'd
like it to work for ARM.

I wrote a simple assembly test case which I'm working on porting it to the
kvm-unit-tests framework. In the non-icount case, I saw roughly the same order
of magnitude for guest IPC before and after the patch. I'd like to also write
CPU frequency (guest CPU cycles per generic timer guest seconds) and (M)IPS
(guest instructions per generic timer guest seconds) tests.

Thanks,
Christopher Covington

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