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

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Message-ID: <54E74E77.2070503@redhat.com>
Date:	Fri, 20 Feb 2015 16:10:47 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Alexander Graf <agraf@...e.de>,
	Bogdan Purcareata <bogdan.purcareata@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-rt-users@...r.kernel.org
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 16:06, Sebastian Andrzej Siewior wrote:
> On 02/20/2015 03:57 PM, Paolo Bonzini wrote:

>> Yes, but large latencies just mean the code has to be rewritten (x86
>> doesn't anymore do event injection in an atomic regions for example).
>> Until it is, using raw_spin_lock is correct.
> 
> It does not sound like it. It sounds more like disabling interrupts to
> get things run faster and then limit it on a different corner to not
> blow up everything.

"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".

> Max latencies was decreased "Max latency (us)  70        62" and that
> is why this is done? For 8 us and possible DoS in case there are too
> many cpus?

My understanding is that:

1) netperf can get you a BUG KVM, and raw_spinlock fixes that

2) cyclictest did not trigger the BUG, and you can also get reduced
latency from using raw_spinlock.

I think we agree that (2) is not a factor in accepting the patch.

Paolo

>> Paolo
>>
> 
> Sebastian
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