From b212b793d0463f1eaa3f99f2c6b4e4f450f48653 Mon Sep 17 00:00:00 2001
From: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Date: Fri, 24 Apr 2015 15:53:13 +0000
Subject: [PATCH 153/158] powerpc/kvm: Disable in-kernel MPIC emulation for
 PREEMPT_RT

While converting the openpic emulation code to use a raw_spinlock_t enables
guests to run on RT, there's still a performance issue. For interrupts sent in
directed delivery mode with a multiple CPU mask, the emulated openpic will loop
through all of the VCPUs, and for each VCPUs, it call IRQ_check, which will loop
through all the pending interrupts for that VCPU. This is done while holding the
raw_lock, meaning that in all this time the interrupts and preemption are
disabled on the host Linux. A malicious user app can max both these number and
cause a DoS.

This temporary fix is sent for two reasons. First is so that users who want to
use the in-kernel MPIC emulation are aware of the potential latencies, thus
making sure that the hardware MPIC and their usage scenario does not involve
interrupts sent in directed delivery mode, and the number of possible pending
interrupts is kept small. Secondly, this should incentivize the development of a
proper openpic emulation that would be better suited for RT.

Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/powerpc/kvm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

Index: linux-5.15.19-rt29/arch/powerpc/kvm/Kconfig
===================================================================
--- linux-5.15.19-rt29.orig/arch/powerpc/kvm/Kconfig
+++ linux-5.15.19-rt29/arch/powerpc/kvm/Kconfig
@@ -178,6 +178,7 @@ config KVM_E500MC
 config KVM_MPIC
 	bool "KVM in-kernel MPIC emulation"
 	depends on KVM && E500
+	depends on !PREEMPT_RT
 	select HAVE_KVM_IRQCHIP
 	select HAVE_KVM_IRQFD
 	select HAVE_KVM_IRQ_ROUTING