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

[Narayana Murty N <nnmlinux@linux.ibm.com>]

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
>