Re: [Linux PPC] Disable PREEMPT

[Christian Zigotzky <chzigotzky@xenosoft.de>]

On 23/09/26 07:13, Shrikanth Hegde wrote:
>
>
> On 9/23/26 10:39 AM, Christian Zigotzky wrote:
>> On 21/09/26 07:12, Shrikanth Hegde wrote:
>>> Hi Christian.
>>>
>>> On 9/20/26 6:34 PM, Christian Zigotzky wrote:
>>>> On 09/20/2026 01:49 PM, Christian Zigotzky wrote:
>>>>> On 09/18/2026 09:10 AM, Michal Suchánek wrote:
>>>>>> On Fri, Sep 18, 2026 at 04:39:57AM +0200, Christian Zigotzky wrote:
>>>>>>> On 17/09/26 16:15, Shrikanth Hegde wrote:
>>>>>>>> Hi Michal,
>>>>>>>>
>>>>>>>>>> Give it a try.
>>>>>>>>>>
>>>>>>>>>> I am suspecting both lockup, stack corruptions are secondary
>>>>>>>>>> symptoms here.
>>>>>>>>>> While recreate, I have different type of traces.
>>>>>>>>> Yes, it does work for the crash I see.
>>>>>>>> Thanks. Give me sometime to see further on that. Didn't get any 
>>>>>>>> time
>>>>>>>> today.
>>>>>>>>
>>>>>>>>> However, we hijacked the report
>>>>>>>>> of KVM locking up on FSL Cyrus+, and that is likely a 
>>>>>>>>> different problem
>>>>>>>>> still not addressed.
>>>>>>>>>
>>>>>>>> Ah, that's what you meant. Yes, i still don't know where FSL 
>>>>>>>> Cyrus+ is
>>>>>>>> stuck.
>>>>>>>>
>>>>>>>> I would need help from FSL Cyrus+ to get some logs or is there 
>>>>>>>> a way we
>>>>>>>> can spin of
>>>>>>>> a QEMU guest of it? We should have the log to start somewhere.
>>>>>>>>
>>>>>>>>
>>>>>>> It's difficult to get a log because it freezes and can't write 
>>>>>>> any error
>>>>>>> messages to the log files. This bug is very old. We had this 
>>>>>>> issue some
>>>>>>> years ago but we was able to disable PREEMPT because we don't 
>>>>>>> need it.
>>>>>> I would expect serial console and sysrq could give something, 
>>>>>> depending
>>>>>> on how much of the kernel is frozen.
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> Michal
>>>>> Here is a serial log: 
>>>>> https://github.com/user-attachments/files/32433586/e5500_preempt_putty.log
>>>>>
>>>
>>> Thanks Michal for helping to get the traces.
>>>
>>>> Another serial log with pcie_ports=compat: 
>>>> https://github.com/user-attachments/files/32434487/e5500_preempt_pcie_ports_compat_putty.log
>>>>
>>>
>>> I see it is mostly busy with printing
>>> WARNING: at .enable_kernel_fp+0x30/0x78, CPU#3: qemu-system-ppc/4884
>>>
>>> The path seems to be same also.
>>>
>>> [c000000085ca7700] [c00000000005de84] .kvmppc_load_guest_fp+0x30/0x80
>>> [c000000085ca7780] [c00000000005f2a0] .kvmppc_handle_exit+0x5bc/0x5cc
>>> [c000000085ca7830] [c00000000006204c] .kvmppc_resume_host+0xb8/0x10c
>>>
>>>
>>> Does below help to boot or make it stable?
>>> ---
>>>  arch/powerpc/kvm/booke.c | 9 ++++++++-
>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
>>> index 13ad4cf5fa71..d4be7a306bfc 100644
>>> --- a/arch/powerpc/kvm/booke.c
>>> +++ b/arch/powerpc/kvm/booke.c
>>> @@ -1404,10 +1404,17 @@ int kvmppc_handle_exit(struct kvm_vcpu 
>>> *vcpu, unsigned int exit_nr)
>>>          if (s <= 0)
>>>              r = (s << 2) | RESUME_HOST | (r & RESUME_FLAG_NV);
>>>          else {
>>> -            /* interrupts now hard-disabled */
>>> +            /*
>>> +             * interrupts now hard-disabled. i.e EE=0
>>> +             * But irqs_disabled relies on PACA entries
>>> +             * and fixup below enables it.
>>> +             * So irqs_disabled == false
>>> +             */
>>>              kvmppc_fix_ee_before_entry();
>>> +            preempt_disable();
>>>              kvmppc_load_guest_fp(vcpu);
>>>              kvmppc_load_guest_altivec(vcpu);
>>> +            preempt_enable_no_resched();
>>>          }
>>>      }
>>>
>>>
>> Hi,
>>
>> Thanks a lot for your patch!
>>
>> I patched the 7.3-rc4 kernel sources with your new patch and compiled 
>> the kernel again yesterday. [1]
>>
>
> Thanks for trying.
>
>> Unfortunately, the issue still exists.
>>
>
> Can you share the console log of the kernel with the patch applied?
> We need to see where it is stuck now. Has it made forward progress?
>
>> I tested it with a guest kernel with and without preempt support.
>>
>> Cheers,
>> Christian
>>
>> [1] 
>> https://github.com/chzigotzky/kernels/releases/tag/v7.3.0-rc4-2-preempt
>>
>
Here is a new serial log from the patched kernel: 
https://github.com/user-attachments/files/32570808/e5500_preempt_pcie_ports_compat_patched_putty.log

Thanks

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