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

[Christopher Covington]

On 09/29/2015 10:07 AM, Christopher Covington wrote:
> 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.

I've sent out the CPI test case and while exercising it I noticed that
Laurent's patch fixed -icount. So my original goal has been accomplished. I'm
happy to rebase this patch if the authorities (Peter Maydell?) think using
cpu_get_ticks() is the right thing to do. Otherwise I'll probably try to move
on to support for the instructions event in the ARM PMU.

Thanks,
Christopher Covington

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