Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux

From: Alexander Graf <[email-masked]>
Date: Fri, 20 Feb 2015 14:45:16 +0100

On 18.02.15 10:32, Bogdan Purcareata wrote:
> This patchset enables running KVM SMP guests with external interrupts on an
> underlying RT-enabled Linux. Previous to this patch, a guest with in-kernel MPIC
> emulation could easily panic the kernel due to preemption when delivering IPIs
> and external interrupts, because of the openpic spinlock becoming a sleeping
> mutex on PREEMPT_RT_FULL Linux.
> 
> 0001: converts the openpic spinlock to a raw spinlock, in order to circumvent
> this behavior. While this change is targeted for a RT enabled Linux, it has no
> effect on upstream kvm-ppc, so send it upstream for better future maintenance.
> 
> 0002: introduces a limit on the maximum VCPUs a guest can have, in order to
> prevent potential DoS attack due to large system latencies. This patch is
> targeted to RT (due to CONFIG_PREEMPT_RT_FULL), but it can also be applied on
> upstream Linux, with no effect. Not sure if it's best to send it upstream and
> have a hanging CONFIG_PREEMPT_RT_FULL check there, with no effect, or send it
> against linux-stable-rt. Please apply as you consider appropriate.

Thomas, what is the usual approach for patches like this? Do you take
them into your rt tree or should they get integrated to upstream?

Alex