Re: [PATCH 2/3] KVM: send IPI to vcpu only when it's in guest mode

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

On 01/05/2011 05:48 PM, Avi Kivity wrote:
> On 01/05/2011 11:38 AM, Xiao Guangrong wrote:
>> We can interrupt the vcpu only when it's running in guest mode
>> to reduce IPI.
>>
>> It looks like only ia64 and x86 need to send IPI to other vcpus, so
>> i only add the implementation of 'vcpu->guest_mode' in ia64, but i
>> don't know ia64 well, please point out the right way for me if the
>> implementation is incorrect
>>
>> And ia64 is not tested since i don't have ia64 box
>>
>> Signed-off-by: Xiao Guangrong<xiaoguangrong@cn.fujitsu.com>
>> ---
>>   arch/ia64/kvm/kvm-ia64.c |    2 ++
>>   virt/kvm/kvm_main.c      |    3 ++-
>>   2 files changed, 4 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
>> index 70d224d..15c11b2 100644
>> --- a/arch/ia64/kvm/kvm-ia64.c
>> +++ b/arch/ia64/kvm/kvm-ia64.c
>> @@ -662,6 +662,7 @@ again:
>>           goto vcpu_run_fail;
>>
>>       srcu_read_unlock(&vcpu->kvm->srcu, idx);
>> +    atomic_set(&vcpu->guest_mode, 1);
>>       kvm_guest_enter();
> 
> I think it needs an smp_wmb() (technically x86 needs it too, but x86 is strongly ordered)
> 

Hi Avi,

After read the code of ia64 more carefully, i noticed ia64 doesn't check 'vcpu->requests'
before go to guest mode, so it means we no need memory barrier here? right?

Certainly, we need an smb_mb between setting vcpu->mode and checking vcpu->request in x86.