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

From: Alexander Graf <[email-masked]>
Date: Fri, 20 Feb 2015 14:45:08 +0100

On 18.02.15 10:32, 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.
> 
> Signed-off-by: Mihai Caraman <[email-masked]>
> Signed-off-by: Bogdan Purcareata <[email-masked]>
> Reviewed-by: Scott Wood <[email-masked]>

I don't think this patch is reasonable to take upstream. If we have a
latency issue, whoever spawned KVM VMs made a decision to spawn such big
VMs.

I'd say let's leave it at MAX_VCPUS == NR_CPUS for now and rather get
someone to resolve any locking problems for real ;).

Alex

> ---
>  arch/powerpc/include/asm/kvm_host.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> index 8ef0512..6f6b928 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -36,8 +36,14 @@
>  #include <asm/cacheflush.h>
>  #include <asm/hvcall.h>
>  
> +#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_KVM_MPIC)
> +/* Limit the number of vcpus due to in-kernel mpic concurrency */
> +#define KVM_MAX_VCPUS		4
> +#define KVM_MAX_VCORES		4
> +#else
>  #define KVM_MAX_VCPUS		NR_CPUS
>  #define KVM_MAX_VCORES		NR_CPUS
> +#endif
>  #define KVM_USER_MEM_SLOTS 32
>  #define KVM_MEM_SLOTS_NUM KVM_USER_MEM_SLOTS
>  
>