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

[Waiman Long <waiman.long@hp.com>]

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@hp.com>
>> ---
>>   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