Re: [Xen-devel] [PATCH RFC v5 5/8] pvqspinlock, x86: Enable unfair queue spinlock in a KVM guest

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

On 02/26/2014 08:44 PM, Waiman Long wrote:
> This patch adds a KVM init function to activate the unfair queue
> spinlock in a KVM guest when the PARAVIRT_UNFAIR_LOCKS kernel config
> option is selected.
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
>   arch/x86/kernel/kvm.c |   17 +++++++++++++++++
>   1 files changed, 17 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 713f1b3..a489140 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -826,3 +826,20 @@ static __init int kvm_spinlock_init_jump(void)
>   early_initcall(kvm_spinlock_init_jump);
>
>   #endif        /* CONFIG_PARAVIRT_SPINLOCKS */
> +
> +#ifdef CONFIG_PARAVIRT_UNFAIR_LOCKS
> +/*
> + * Enable unfair lock if running in a real para-virtualized environment
> + */
> +static __init int kvm_unfair_locks_init_jump(void)
> +{
> +       if (!kvm_para_available())
> +               return 0;
> +
kvm_kick_cpu_type() in patch 8 assumes that host has support for kick
hypercall (KVM_HC_KICK_CPU).

I think for that we need explicit check of this 
kvm_para_has_feature(KVM_FEATURE_PV_UNHALT).
otherwise things may break for unlikely case of running a new guest on
a old host?


> +       static_key_slow_inc(&paravirt_unfairlocks_enabled);
> +       printk(KERN_INFO "KVM setup unfair spinlock\n");
> +
> +       return 0;
> +}
> +early_initcall(kvm_unfair_locks_init_jump);
> +#endif

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