Re: [Linux PPC] Disable PREEMPT

[Christian Zigotzky <chzigotzky@xenosoft.de>]

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]

Unfortunately, the issue still exists.

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

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