linux-kernel - Re: [PATCH 2/2] powerpc/kvm: Limit MAX_VCPUS for guests running on RT Linux

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Message-ID: <54E45D31.3020302@linutronix.de>
Date:	Wed, 18 Feb 2015 10:36:49 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Bogdan Purcareata <bogdan.purcareata@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-rt-users@...r.kernel.org,
	agraf@...e.de, pbonzini@...hat.com
CC:	linux-kernel@...r.kernel.org, scottwood@...escale.com,
	mihai.caraman@...escale.com
Subject: Re: [PATCH 2/2] powerpc/kvm: Limit MAX_VCPUS for guests running on
 RT Linux

On 02/18/2015 10:32 AM, Bogdan Purcareata wrote:
> Due to the introduction of the raw_spinlock for the KVM openpic, guests with a
> high number of VCPUs may induce great latencies on the underlying RT Linux
> system (e.g. cyclictest reports latencies of ~15ms for guests with 24 VCPUs).
> This can be further aggravated by sending a lot of external interrupts to the
> guest.
> 
> A malicious app can abuse this scenario, causing a DoS of the host Linux.
> Until the KVM openpic code is refactored to use finer lock granularity, impose
> a limitation on the number of VCPUs a guest can have when running on a
> PREEMPT_RT_FULL system with KVM_MPIC emulation.

How is this possible? You take the raw lock, write a register, release
the raw lock. How can the guest lockup the host? Is this write blocking
in guest?

Sebastian
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