Re: [Linux PPC] Disable PREEMPT

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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();
  		}
  	}