Re: [Linux PPC] Disable PREEMPT

[Christian Zigotzky <chzigotzky@xenosoft.de>]

On 03 September 2026 at 07:23 Shrikanth Hegde <sshegde@linux.ibm.com> wrote:

Friendly LLM analysis says preemption is enabled too early and before
completing kvmppc_handle_exit_hv() / kvmppc_handle_nested_exit()

Below is ONLY a speculation and completely UNTESTED.
Maybe worth a try.

---
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 0409ac9e7b31..78d2b0618887 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -5049,8 +5049,6 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,

       powerpc_local_irq_pmu_restore(flags);

-       preempt_enable();
-
       /*
        * cancel pending decrementer exception if DEC is now positive, or if
        * entering a nested guest in which case the decrementer is now owned
@@ -5072,6 +5070,8 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
       }
       vcpu->arch.ret = r;

+       preempt_enable();
+
       if (is_kvmppc_resume_guest(r) && !kvmppc_vcpu_check_block(vcpu)) {
               kvmppc_set_timer(vcpu);


Michal

- - -

Michal,

Is this patch also for the issues with KVM PR and KVM HV if PREEMPT is activated?

The entire FSL Cyrus+ board freezes when using KVM HV with PREEMPT.

The guest kernel doesn't boot if we use KVM PR with a PREEMPT kernel on
the PA Semi Nemo board.

Bug report: https://github.com/chzigotzky/kernels/issues/19

Thanks,
Christian