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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

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>