Subject: powerpc/kvm: Disable in-kernel MPIC emulation for PREEMPT_RT From: Bogdan Purcareata <bogdan.purcareata@freescale.com> Date: Fri Apr 24 15:53:13 2015 +0000 From: Bogdan Purcareata <bogdan.purcareata@freescale.com> 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-6.8.2-rt10/arch/powerpc/kvm/Kconfig =================================================================== @ linux-6.8.2-rt10/arch/powerpc/kvm/Kconfig:225 @ config KVM_E500MC config KVM_MPIC bool "KVM in-kernel MPIC emulation" depends on KVM && PPC_E500 + depends on !PREEMPT_RT select HAVE_KVM_IRQCHIP select HAVE_KVM_IRQ_ROUTING select HAVE_KVM_MSI