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

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Date:	Thu, 26 Feb 2015 14:02:38 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Scott Wood <scottwood@...escale.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	Alexander Graf <agraf@...e.de>,
	Bogdan Purcareata <bogdan.purcareata@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-rt-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, mihai.caraman@...escale.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux



On 24/02/2015 00:27, Scott Wood wrote:
> This isn't a host PIC driver.  It's guest PIC emulation, some of which
> is indeed not suitable for a rawlock (in particular, openpic_update_irq
> which loops on the number of vcpus, with a loop body that calls
> IRQ_check() which loops over all pending IRQs).

The question is what behavior is wanted of code that isn't quite
RT-ready.  What is preferred, bugs or bad latency?

If the answer is bad latency (which can be avoided simply by not running
KVM on a RT kernel in production), patch 1 can be applied.  If the
answer is bugs, patch 1 is not upstream material.

I myself prefer to have bad latency; if something takes a spinlock in
atomic context, that spinlock should be raw.  If it hurts (latency),
don't do it (use the affected code).

Paolo

> The vcpu limits are a
> temporary bandaid to avoid the worst latencies, but I'm still skeptical
> about this being upstream material.

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