Re: [PATCH v3] KVM: PPC: Book3S HV: Avoid spurious interrupts caused by LPCR_MER bit

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi Gautam,

Thanks for the patch and working on this.  I have a couple of questions though.

On 2026/09/21 04:40 PM, Gautam Menghani wrote:
> A huge number of spurious interrupts can be seen immediately after a KVM
> on PowerNV guest boots up in XIVE mode.
> 
> $ cat /proc/interrupts  | grep SPU
> SPU:     223705     192439     273526     147623   Spurious interrupts
> 
> This bug was introduced by commit ecd10702baae5 ("KVM: PPC: Book3S HV:
> Handle pending exceptions on guest entry with MSR_EE"). The root cause
> is that once LPCR_MER bit is set, it is supposed to be reset by
> software. But when a vCPU starts running with LPCR_MER set, the vCPU does
> not exit back to the host until the decrementer expires or there is an
> hcall, etc. This is because KVM on PowerNV guests have support for
> native XIVE, so they are not dependent on host for interrupt emulation.
> Due to this behaviour, a huge number of spurious interrupts are seen
> since LPCR_MER continues to be set and LPCR_MER cannot be reset until the
> vCPU exits to the host.
> 
> Fix this behaviour by not using the LPCR_MER bit whenever native XIVE is
> available (currently in case of KVM on PowerNV only), as the XIVE hardware
> can present interrupts to the KVM guest vCPU directly. So the LPCR_MER
> functionality is not required. This reduces the number of spurious
> interrupts drastically.
> 
> Fixes: ecd10702baae5 ("KVM: PPC: Book3S HV: Handle pending exceptions on guest entry with MSR_EE")
> Cc: stable@vger.kernel.org # 6.8+
> Reported-by: Timothy Pearson <tpearson@raptorengineering.com>
> Closes: https://lore.kernel.org/linuxppc-dev/582904882.11159.1786719390349.JavaMail.zimbra@raptorengineeringinc.com
> Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
> ---
> v3:
> 1. Continue the use of LPCR_MER when kernel-irqchip=off (Sashiko)
> 
> v2:
> 1. Handle the case where xive_interrupt_pending() is true and also the
> external exception bit is set. (Narayana)
> 
>  arch/powerpc/include/asm/kvm_ppc.h | 7 +++++++
>  arch/powerpc/kvm/book3s_hv.c       | 2 +-
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
> index 169ea6a7fbad..580ad2548c2b 100644
> --- a/arch/powerpc/include/asm/kvm_ppc.h
> +++ b/arch/powerpc/include/asm/kvm_ppc.h
> @@ -747,6 +747,11 @@ static inline int kvmppc_xive_enabled(struct kvm_vcpu *vcpu)
>  	return vcpu->arch.irq_type == KVMPPC_IRQ_XIVE;
>  }
>  
> +static inline bool kvmppc_xive_native_enabled(struct kvm *kvm)
> +{
> +	return kvm->arch.xive_devices.native;
> +}
> +
>  extern int kvmppc_xive_native_connect_vcpu(struct kvm_device *dev,
>  					   struct kvm_vcpu *vcpu, u32 cpu);
>  extern void kvmppc_xive_native_cleanup_vcpu(struct kvm_vcpu *vcpu);
> @@ -782,6 +787,8 @@ static inline bool kvmppc_xive_rearm_escalation(struct kvm_vcpu *vcpu) { return
>  
>  static inline int kvmppc_xive_enabled(struct kvm_vcpu *vcpu)
>  	{ return 0; }
> +static inline bool kvmppc_xive_native_enabled(struct kvm *kvm) { return false; }
> +
>  static inline int kvmppc_xive_native_connect_vcpu(struct kvm_device *dev,
>  			  struct kvm_vcpu *vcpu, u32 cpu) { return -EBUSY; }
>  static inline void kvmppc_xive_native_cleanup_vcpu(struct kvm_vcpu *vcpu) { }
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index dbac3573b2c8..cb2bb29451a7 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -4980,7 +4980,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
>  			if (!kvmhv_on_pseries() && (__kvmppc_get_msr_hv(vcpu) & MSR_EE))
>  				kvmppc_inject_interrupt_hv(vcpu,
>  							   BOOK3S_INTERRUPT_EXTERNAL, 0);
> -			else
> +			else if (!kvmppc_xive_native_enabled(vcpu->kvm))
>  				lpcr |= LPCR_MER;

1. What happens when an L1 KVM guest is booted with (native) XIVE and then the
   guest is rebooted with `xive=off` i.e., with XICS?  Will we stop setting
   LPCR_MER as kvm->arch.xive_devices.native would still be set?
2. What happends when an L1 KVM guest is booted with XIVE and then we kexec into
   a new kernel with `xive=off`?  You did mention in your other reply that with
   kexec, `xive=off` is ignored currently but IMO, we would want to understand
   where this limiation lies and fix that if need be.  I do understand we are
   trying to fix spurious interrupts problem with XIVE in this patch and this
   particular problem can be taken separately but it worth investigating.

Thanks,
Amit