Re: [Qemu-discuss] Imprecise instruction count when compared to real hardware

[Humberto \"SilverOne\" Carvalho]

Hi!

I forgot to mention that the counter is incremented through a HELPER method.

> I'm looking to compute the exact number of executed instructions in Qemu for 
> userland arm binaries.

> To do this, i added my own counter (an unsigned long) on the TCG translator 
> loop in accel/tcg/translator.c.
> The counter is incremented in the translator_loop function on line 69 (next 
> to dv->num_insns++;)

The counter is incremented through a TCG helper method.

> At the end of program execution, the total number of instructions is printed.

> Next, i compile a set of benchmarks and execute them on qemu-arm and cortex 
> A9 platform. I use perf to measure the number of cycles on the arm A9 
> processor.

> Using this method Qemu has an average absolute error of 3%, although the 
> maximum error on one test was -18%. Qemu will generally underestimate the 
> number of executed instructions but can sometimes overestimate by up to 3%.

> My goal is to understand why the number of executed instructions is not 100% 
> accurate.
> I understand TCG performs a couple of optimizations. However, this does not 
> explain the cases where Qemu overestimates, leading me to believe there may 
> be other motives for the lack of accuracy.

Best Regards,
SilverOne