[Linux PPC] KVM PR and KVM HV do not work if the kernel was compiled with PREEMPT

[Christian Zigotzky <chzigotzky@xenosoft.de>]

On 17/02/26 12:34, Shrikanth Hegde wrote:
>
>
> On 2/17/26 3:56 PM, Christian Zigotzky wrote:
>> On 17/02/26 11:17 Madhavan Srinivasan wrote:
>>>
>>> On 2/17/26 3:14 PM, Christian Zigotzky wrote:
>>>> On 17/02/26 10:03, Shrikanth Hegde wrote:
>>>>>
>>>>>
>>>>> On 2/17/26 2:22 PM, Christian Zigotzky wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Thank you for your patch. I tested the latest git kernel with 
>>>>>> your patch today. Unfortunately the system freezes while booting 
>>>>>> the VM with KVM HV.
>>>>>>
>>>>>> Cheers,
>>>>>> Christian
>>>>>>
>>>>>>
>>>>> Would it be possible to share the console log or boot log of KVM 
>>>>> to the
>>>>> point it freezes?
>>>>
>>>> Unfortunately there aren't any error messages.
>>>>
>>>> I also tried it with:
>>>>
>>>> gdb --args qemu-system-ppc64 -M ppce500 -cpu e5500 -m 1024 -kernel 
>>>> uImage-7.0-preempt -drive 
>>>> format=raw,file=void-live-powerpc-20230317- 
>>>> updated2.img,index=0,if=virtio -netdev user,id=mynet0 -device 
>>>> virtio- net,netdev=mynet0 -append "rw root=/dev/vda" -device 
>>>> virtio-gpu-pci - device virtio-mouse-pci -device 
>>>> virtio-keyboard-pci -enable-kvm -smp 4
>>>>
>>>> and with gdb in the guest:
>>>>
>>>> qemu-system-ppc64 -s -S -M ppce500 -cpu e5500 -m 1024 -kernel 
>>>> uImage-7.0-preempt -drive 
>>>> format=raw,file=void-live-powerpc-20230317- 
>>>> updated2.img,index=0,if=virtio -netdev user,id=mynet0 -device 
>>>> virtio- net,netdev=mynet0 -append "rw root=/dev/vda" -device 
>>>> virtio-gpu-pci - device virtio-mouse-pci -device 
>>>> virtio-keyboard-pci -enable-kvm -smp 4
>>>>
>>>> The system freezed without any error messages.
>
> Anything on host console?
>
>>> can you please share the config.
>>> I tried kvm-hv in my power8/power9 and seems to boot fine without 
>>> any issues with today's upstream
>>> Maddy
>> Here are the configs.
>>
>> With PREEMPT: https://github.com/chzigotzky/kernels/blob/ 
>> d50d259ca63dd2b3b9a9eaabc82e5cc69c8cc9f0/configs/e5500_defconfig
>>
>> Without PREEMPT: https://github.com/chzigotzky/kernels/blob/main/ 
>> configs/e5500_defconfig
>>
>> - Christian
>
> I looked at the code. there was one more place where cond_resched was 
> there.
>
> If possible give the below try.
> (Note: Didn;t replicate your issue)
>
> ---
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 7667563fb9ff..bf0ca49c9915 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -4825,10 +4825,13 @@ static int kvmppc_run_vcpu(struct kvm_vcpu *vcpu)
>          vc->runner = vcpu;
>          if (n_ceded == vc->n_runnable) {
>              kvmppc_vcore_blocked(vc);
> -        } else if (need_resched()) {
> +        } else if (need_resched() || spin_needbreak(&vc->lock)) {
>              kvmppc_vcore_preempt(vc);
>              /* Let something else run */
> -            cond_resched_lock(&vc->lock);
> +            spin_unlock(&vc->lock);
> +            schedule();
> +            //cond_resched_lock(&vc->lock);
> +            spin_lock(&vc->lock);
>              if (vc->vcore_state == VCORE_PREEMPT)
>                  kvmppc_vcore_end_preempt(vc);
>          } else {
> @@ -4901,7 +4904,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, 
> u64 time_limit,
>      }
>
>      if (need_resched())
> -        cond_resched();
> +        schedule();
>
>      kvmppc_update_vpas(vcpu);
>
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 9a89a6d98f97..54963c1d8b58 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -86,7 +86,7 @@ int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
>      while (true) {
>          if (need_resched()) {
>              local_irq_enable();
> -            cond_resched();
> +            schedule();
>              hard_irq_disable();
>              continue;
>          }
>
Hello Shrikanth,

I tested your patch today. The patched preempt kernel boots on my e5500 
host and in an e5500 VM. Unfortunately my guest and host freeze after 
loading the VirtIO-GPU.

Thanks for your help,

Christian

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