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

From: Paolo Bonzini <[email-masked]>
Date: Thu, 26 Feb 2015 14:02:38 +0100

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.