[Qemu-devel] emulation cycle number implementation

[Qemu-devel] emulation cycle number implementation

[Sergey Smolov]

Hello, List!


I'm running MIPS assembler programs on QEMU. I need to log some events 
that happen through emulation (writes to some registers, for example). 
When an event happens, I need to print not only the event-related 
information, but an "emulation cycle number" also. This number should be 
an integer increasing value that is similar to real hardware clock.

How to implement this in QEMU? May QEMU timers be helpful in solving 
such a problem?

Thanks in advance.

-- 
Sincerely yours,
Sergey Smolov

Re: [Qemu-devel] emulation cycle number implementation

[Peter Maydell]

On 7 September 2017 at 17:30, Sergey Smolov <smolov@ispras.ru> wrote:
> I'm running MIPS assembler programs on QEMU. I need to log some events that
> happen through emulation (writes to some registers, for example). When an
> event happens, I need to print not only the event-related information, but
> an "emulation cycle number" also. This number should be an integer
> increasing value that is similar to real hardware clock.
>
> How to implement this in QEMU? May QEMU timers be helpful in solving such a
> problem?

You can use cpu_get_ticks(), though if we're not in icount mode
you may find it doesn't behave quite as helpfully as you might like
(in that case it roughly tracks host hardware cycles.)

thanks
-- PMM

Re: [Qemu-devel] emulation cycle number implementation

[Sergey Smolov]

On 07.09.2017 19:37, Peter Maydell wrote:
> On 7 September 2017 at 17:30, Sergey Smolov <smolov@ispras.ru> wrote:
>> I'm running MIPS assembler programs on QEMU. I need to log some events that
>> happen through emulation (writes to some registers, for example). When an
>> event happens, I need to print not only the event-related information, but
>> an "emulation cycle number" also. This number should be an integer
>> increasing value that is similar to real hardware clock.
>>
>> How to implement this in QEMU? May QEMU timers be helpful in solving such a
>> problem?
> You can use cpu_get_ticks(), though if we're not in icount mode
> you may find it doesn't behave quite as helpfully as you might like
> (in that case it roughly tracks host hardware cycles.)
>
> thanks
> -- PMM

Thank you, Peter!

But what if I like to track guest hardware cycles? I run the same 
program on QEMU and RTL model of MIPS microprocessor, and I want to 
compare simulation traces. I expect, that emulation will start from 0 
tick in both cases, at least.

-- 
Sincerely yours,
Sergey Smolov

Re: [Qemu-devel] emulation cycle number implementation

[Peter Maydell]

On 8 September 2017 at 08:41, Sergey Smolov <smolov@ispras.ru> wrote:
> But what if I like to track guest hardware cycles? I run the same program on
> QEMU and RTL model of MIPS microprocessor, and I want to compare simulation
> traces. I expect, that emulation will start from 0 tick in both cases, at
> least.

Broadly, you can't, because QEMU is not a cycle accurate emulator.
(For 21st century CPUs, cycle accurate emulation that isn't just
"run the RTL" is pretty near to impossible for practical purposes
anyway.) If you're using QEMU's icount mode then cpu_get_ticks()
will I think get you number of instructions executed; that's as
good as you're going to get, I'm afraid.

Last time I wanted to do comparison of simulation traces I just
compared the sequence of instructions executed, without worrying
about trying to compare against the other simulation's notional
clock cycle count as well.

thanks
-- PMM