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

[Waiman Long <waiman.long@xxxxxx>]

On 02/27/2014 05:40 AM, Raghavendra K T wrote:
> 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?
Unfair lock is a separate config option that does not need to do any cpu 
kick. The checking of kvm_para_available() is just to make sure that the 
kernel is running in a real PV environment, not on bare metal but with 
CONFIG_PARAVIRT enabled.
-Longman

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