Re: [Linux PPC] Disable PREEMPT

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

>> If possible run against current upstream and share the results.
> 
> https://github.com/openSUSE/kernel-source/blob/6824496d1801f73def615dca8794202eeb7b0d86/config/ppc64le/default
> 
> [  472.091531][ T6181] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: kvmhv_run_single_vcpu+0x19d4/0x1b50 [kvm_hv]
> [  472.091598][ T6181] CPU: 29 UID: 107 PID: 6181 Comm: CPU 112/KVM Not tainted 7.2.2-5.g6824496-default #1 PREEMPT(full) openSUSE Tumbleweed (unreleased)  61871a5f06863b4006f5ef27cd9f18e8a7a2edad
> [  472.091612][ T6181] Hardware name: IBM,9824-42A Power11 (architected) 0x820200 0xf000007 of:IBM,FW1110.20 (OB1110_130) hv:phyp pSeries
> [  472.091624][ T6181] Call Trace:
> [  472.091630][ T6181] [c00000000fdfb680] [c00000000134ce90] dump_stack_lvl+0x84/0xc0 (unreliable)
> [  472.091653][ T6181] [c00000000fdfb6b0] [c00000000022e7c8] vpanic+0x324/0x5e4
> [  472.091666][ T6181] [c00000000fdfb760] [c00000000022eac4] do_panic_on_target_cpu+0x0/0x2c
> [  472.091677][ T6181] [c00000000fdfb780] [c0000000013c1ff8] __stack_chk_fail+0x48/0x60
> [  472.091689][ T6181] [c00000000fdfb7f0] [c00800001aae219c] kvmhv_run_single_vcpu+0x19d4/0x1b50 [kvm_hv]
> [  472.091712][ T6181] [c00000000fdfb940] [c00800001aae24b4] kvmppc_vcpu_run_hv+0x19c/0x12f0 [kvm_hv]
> [  472.091732][ T6181] [c00000000fdfba10] [c00800001aeeed18] kvmppc_vcpu_run+0x30/0x48 [kvm]
> [  472.091779][ T6181] [c00000000fdfba30] [c00800001aee9ef4] kvm_arch_vcpu_ioctl_run+0x35c/0x4a0 [kvm]
> [  472.091813][ T6181] [c00000000fdfbac0] [c00800001aedaac4] kvm_vcpu_ioctl+0x1ac/0xad8 [kvm]
> [  472.091844][ T6181] [c00000000fdfbca0] [c0000000007f1244] sys_ioctl+0x374/0x1060
> [  472.091857][ T6181] [c00000000fdfbdb0] [c00000000002f7f8] system_call_exception+0x188/0x430
> [  472.091871][ T6181] [c00000000fdfbe50] [c00000000000cfdc] system_call_vectored_common+0x15c/0x2ec
> [  472.091886][ T6181] ---- interrupt: 3000 at 0x7fffb5565fac
> [  472.091896][ T6181] NIP:  00007fffb5565fac LR: 00007fffb5565fac CTR: 0000000000000000
> [  472.091904][ T6181] REGS: c00000000fdfbe80 TRAP: 3000   Not tainted  (7.2.2-5.g6824496-default)
> [  472.091911][ T6181] MSR:  800000000280f033 <SF,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE>  CR: 42044402  XER: 00000000
> [  472.091938][ T6181] IRQMASK: 0
> [  472.091938][ T6181] GPR00: 0000000000000036 00007fbfa77ed7a0 00007fffb5677100 00000000000000fa
> [  472.091938][ T6181] GPR04: 000000002000ae80 0000000000000000 0000000000000000 0000000000000000
> [  472.091938][ T6181] GPR08: 00000000000000fa 0000000000000000 0000000000000000 0000000000000000
> [  472.091938][ T6181] GPR12: 0000000000000000 00007fbfa77f5ec0 000000014676f000 00007fbfa77ee7c0
> [  472.091938][ T6181] GPR16: 000000014674e8d0 00007fbfa77eeec0 00007fbfa77eeec0 fffffffffffffff7
> [  472.091938][ T6181] GPR20: 00007fffb71210d0 0000000000000001 00007fbfa77eeec0 0000000000000000
> [  472.091938][ T6181] GPR24: 00007fbfa77ed8e8 0000000105971428 000000002000ae80 0000000105f77a70
> [  472.091938][ T6181] GPR28: 0000000000000000 0000000000000000 000000002000ae80 000000014674f000
> [  472.092020][ T6181] NIP [00007fffb5565fac] 0x7fffb5565fac
> [  472.092027][ T6181] LR [00007fffb5565fac] 0x7fffb5565fac
> [  472.092033][ T6181] ---- interrupt: 3000
> [  472.098256][ T6181] pstore: backend (nvram) writing error (-1)
> 
> This is the host, cannot run the kernel as guest because it fails to boot most
> of the time inside KVM.
> 
>>
>>> Nonethless, there are quite a few platforms. Originally no preemption
>>> was the only option, and that's the reason why many people run that.
>>> It's the conservative, known working option. And that's the reason a lot
>>> of platfrom code does not get tested with more aggressive preemtion
>>> models, and never gets fixed to work with them.
>>>
>>
>> Full preemption has been there for many years!.
> 
> Possible for years, forced only recently.
> 
>> Lazy is not that aggressive compared to that.
>>
>>> Simply disabling the no preemtion option does not make the platform code
>>> ready.
>>>
>>
>> Let's understand your crash case. Let's see where it is going wrong. I am suspecting
>> it is some wrong usage of preemption api rather than arch can't support preemption.
> 
> Very likely some wrong use of the preemption API by the arch code, or no
> use where it should have been used. It did not matter so long as people
> could run their no preempt configs and ignore the problem.
> 
> Thanks


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