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

From: Paolo Bonzini
Date: Thu Feb 26 2015 - 08:03:15 EST




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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/