[PATCH v5] KVM: PPC: Book3S HV: Avoid spurious interrupts caused by LPCR_MER bit

[PATCH v5] KVM: PPC: Book3S HV: Avoid spurious interrupts caused by LPCR_MER bit

[Gautam Menghani]

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>
---
v5:
1. Drop the redundant check for the presence of the native XIVE
device. (Ritesh)

v4:
1. If interrupt mode is changed to XICS during guest reboot, MER should
be used. (Amit)

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/kvm/book3s_hv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 22283d46e8fe..41012b0accf4 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -4984,7 +4984,7 @@ 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_enabled(vcpu))
 				lpcr |= LPCR_MER;
 		} else {
 			/*
-- 
2.55.0

Re: [PATCH v5] KVM: PPC: Book3S HV: Avoid spurious interrupts caused by LPCR_MER bit

[Ritesh Harjani]

Gautam Menghani <gautam@linux.ibm.com> writes:

> 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>
> ---
> v5:
> 1. Drop the redundant check for the presence of the native XIVE
> device. (Ritesh)
>

Thanks Gautam for confirming that.

The patch looks good to me. Feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Re: [PATCH v5] KVM: PPC: Book3S HV: Avoid spurious interrupts caused by LPCR_MER bit

[Narayana Murty N]

Hi Gautam,

Thanks for the update.

On 02/11/26 4:42 PM, Ritesh Harjani (IBM) wrote:
> Gautam Menghani <gautam@linux.ibm.com> writes:
> 
>> 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>
>> ---
>> v5:
>> 1. Drop the redundant check for the presence of the native XIVE
>> device. (Ritesh)
>>
> 
> Thanks Gautam for confirming that.
> 
> The patch looks good to me. Feel free to add:
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Using kvmppc_xive_enabled(vcpu) looks sufficient here since
KVMPPC_IRQ_XIVE specifically represents XIVE native exploitation mode.
It also preserves the required behaviour when switching back to XICS and
when the in-kernel irqchip is not being used.

The earlier case where both BOOK3S_IRQPRIO_EXTERNAL and
xive_interrupt_pending() are set also remains handled by the outer
condition.

Reviewed-by: Narayana Murty N nnmlinux@linux.ibm.com
>