Re: high cpu usage in idle state

[Philippe Mathieu-Daudé]

On 9/19/21 20:06, Peter Maydell wrote:
> On Sat, 18 Sept 2021 at 09:12, Ali Vatankhah <alivatankhah72@gmail.com> wrote:
>> then to check what instructions are executing I run this command:
>>
>> qemu-system-ppc64 -nographic -M ppce500 -cpu e5500 -kernel uImage 
>> -singlestep -d in_asm -D target_asm.log
>>
>> Obviously this results in a Kernel panic, but the point is that after panic 
>> there is no guest
>>  instruction running as there is no more log in the file but still CPU usage 
>> is 100.
> 
> The lack of further in_asm logging does not mean that guest instructions
> are not running. in_asm logging happens at *translate* time,
> which is to say the first time QEMU encounters any particular instruction.
> After translation, QEMU can re-execute the translated code for that
> instruction many times, and it will not show up in in_asm logs again.
> In particular, if the guest CPU is doing either of:
>  * a tight loop
>  * an infinite loop of taking exceptions
> it will just be re-running code that has been seen before.
> Probably the code the kernel runs after it panic()s is just a loop.
> 
> If you want to log execution, you need to add 'exec' and/or 'cpu' to
> your -d logging. (Warning: this can generate a lot of logging output
> and massively slow down execution as a result.)

In the "infinite loop exception" case, '-d int' might be sufficient,
before using 'exec/cpu', since you'll see the exception raise over
and over.

>> also run this command to check generated host assembly code:
>>
>> qemu-system-ppc64 -nographic -M ppce500 -cpu e5500 -kernel uImage 
>> -singlestep -d out_asm -D host_asm.log
> 
> out_asm also is logged only at translate time, not at execution time.
> 
> -- PMM
>