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

[Christopher Covington]

On 10/02/2015 01:25 PM, Peter Crosthwaite wrote:
> On Fri, Oct 2, 2015 at 9:56 AM, Peter Maydell <address@hidden> wrote:
>> On 2 October 2015 at 17:44, Christopher Covington <address@hidden> wrote:
>>> 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.
>>
>> Authority here is probably Peter Crosthwaite. I can produce an
>> opinion but I'd have to go and research a bunch of stuff to do
>> that, so I'm hoping to avoid it...
> 
> So my idea here is the CPU input frequency should be a property of the CPU.
> 
> Some experimental results confirm that the PMCCNTR on many common ARM
> implementations is directly connected to the input clock and can be
> relied on as a straight free-running counter. I think a genuine
> instruction counter is something else

Yes, the "genuine" instruction counter is something else. The instruction
count is only relevant for folks trying to get deterministic execution by
using the -icount option. QEMU TCG mode does not emulate a cycle-level input
clock for the guest (the whole class of functional models skip this
time-consuming step) but rather operates a block at a time. By doing a little
extra, I think it also interpolates the exact instruction count. Specifying a
fixed IPC = n is the most sensible way of deterministically calculating a
PMCCNTR_EL0 value that I know of. The -icount option allows users to choose
such deterministic behavior.

> and this timer should be independent of any core provider of cycle count.

What, if anything, do you think should be hooked up to the core provider of
cycle count?

Christopher Covington

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