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

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Date:	Fri, 20 Feb 2015 15:12:55 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Alexander Graf <agraf@...e.de>,
	Bogdan Purcareata <bogdan.purcareata@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-rt-users@...r.kernel.org,
	bigeasy@...utronix.de
CC:	linux-kernel@...r.kernel.org, scottwood@...escale.com,
	mihai.caraman@...escale.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux



On 20/02/2015 14:45, Alexander Graf wrote:
> 
> 
> 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?

Patch 1 is definitely suitable for upstream, that's the reason why we
have raw_spin_lock vs. raw_spin_unlock.

Paolo
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