Re: [PATCH] KVM: PPC: Book3S HV: Avoid triggering an extra interrupt for a vCPU

[Gautam Menghani]

On Tue, Sep 15, 2026 at 02:08:28PM +0530, Narayana Murty N wrote:
> Hi Gautam,
> 
> On 15/09/26 11:31 AM, 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 when there is an interrupt pending for a vCPU
> > (xive_interrupt_pending() returns true) and MSR_EE is disabled for the
> > vCPU, LPCR_MER ends up getting set for the VCPU. When the vCPU starts
> > running, the XIVE hardware presents the pending interrupt to the vCPU
> > (since the KVM guest has native XIVE support) and then a second spurious
> > interrupt gets presented to the vCPU due to the LPCR_MER bit being set.
> I agree that we should avoid setting LPCR_MER when the pending interrupt
> will already be delivered natively by XIVE on PowerNV.
> > 
> > Fix this behaviour by not queuing up any extra interrupts with LPCR_MER
> > if there is an interrupt already pending in the case of KVM on PowerNV.
> > 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@xxxxxxxxxxxxxxx #6.8+
> > Reported-by: Timothy Pearson <tpearson@xxxxxxxxxxxxxxxxxxxxx>
> > Closes: https://lore.kernel.org/linuxppc-dev/582904882.11159.1786719390349.JavaMail.zimbra@xxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>
> > ---
> >   arch/powerpc/kvm/book3s_hv.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index 7667563fb9ff..fda3767ebc9e 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -4937,9 +4937,8 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
> >   	if (!nested) {
> >   		kvmppc_core_prepare_to_enter(vcpu);
> The current code handles two different indications of an external interrupt:
> > -		if (test_bit(BOOK3S_IRQPRIO_EXTERNAL,
> > -			     &vcpu->arch.pending_exceptions) ||
> > -		    xive_interrupt_pending(vcpu)) {
> > +		if (!xive_interrupt_pending(vcpu) && test_bit(BOOK3S_IRQPRIO_EXTERNAL,
> > +							      &vcpu->arch.pending_exceptions)) {
> >   			/*
> >   			 * For nested HV, don't synthesize but always pass MER,
> >   			 * the L0 will be able to optimise that more
> This means xive_interrupt_pending() now gates the entire block, including
> the BOOK3S_IRQPRIO_EXTERNAL handling, rather than only avoiding the LPCR_MER
> which causes the duplicate interrupt.

Yes valid point. If both conditions are true, we'll never get inside the
if block, which is undesirable. I'll fix this in v2.

> 
> Also, the changelog describes the issue as specific to KVM on PowerNV, while
> this condition also affects the pSeries/nested handling in the same block.

No, xive_interrupt_pending() always returns false for KVM on Pseries. So
effectively, we end up only checking for the BOOK3S_IRQPRIO_EXTERNAL
bit. So this is not a problem for KVM on LPAR / nested guests on
PowerNV.

> 
> Thanks,
> Narayana Murty N