Re: [PATCH] KVM: Get rid of KVM_REQ_KICK

[Avi Kivity <avi@redhat.com>]

On 05/04/2010 07:31 PM, Marcelo Tosatti wrote:
> On Mon, May 03, 2010 at 05:19:08PM +0300, Avi Kivity wrote:
>    
>> KVM_REQ_KICK poisons vcpu->requests by having a bit set during normal
>> operation.  This causes the fast path check for a clear vcpu->requests
>> to fail all the time, triggering tons of atomic operations.
>>      
> Avi,
>
> Do you have numbers?
>    

Forgot to post, was about 100 cycles (I expected more, all those atomics 
really show up in the profile).

>> Fix by replacing KVM_REQ_KICK with a vcpu->guest_mode atomic.
>>
>> Signed-off-by: Avi Kivity<avi@redhat.com>
>> ---
>>   arch/x86/kvm/x86.c       |   17 ++++++++++-------
>>   include/linux/kvm_host.h |    1 +
>>   2 files changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 6b2ce1d..307094a 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4499,13 +4499,15 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>   	if (vcpu->fpu_active)
>>   		kvm_load_guest_fpu(vcpu);
>>
>> -	local_irq_disable();
>> +	atomic_set(&vcpu->guest_mode, 1);
>> +	smp_wmb();
>>      
> IPI can trigger here?
>    

It can...

>    
>> -	clear_bit(KVM_REQ_KICK,&vcpu->requests);
>> -	smp_mb__after_clear_bit();
>> +	local_irq_disable();
>>
>> -	if (vcpu->requests || need_resched() || signal_pending(current)) {
>> -		set_bit(KVM_REQ_KICK,&vcpu->requests);
>> +	if (!atomic_read(&vcpu->guest_mode) || vcpu->requests
>> +	    || need_resched() || signal_pending(current)) {
>>      

... and we'll detect that guest_mode was cleared and go back.

>> +		atomic_set(&vcpu->guest_mode, 0);
>> +		smp_wmb();
>>   		local_irq_enable();
>>   		preempt_enable();
>>   		r = 1;
>> @@ -4550,7 +4552,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>   	if (hw_breakpoint_active())
>>   		hw_breakpoint_restore();
>>
>> -	set_bit(KVM_REQ_KICK,&vcpu->requests);
>> +	atomic_set(&vcpu->guest_mode, 0);
>> +	smp_wmb();
>>   	local_irq_enable();
>>
>>   	++vcpu->stat.exits;
>> @@ -5470,7 +5473,7 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
>>
>>   	me = get_cpu();
>>   	if (cpu != me&&  (unsigned)cpu<  nr_cpu_ids&&  cpu_online(cpu))
>> -		if (!test_and_set_bit(KVM_REQ_KICK,&vcpu->requests))
>> +		if (atomic_xchg(&vcpu->guest_mode, 0))
>>   			smp_send_reschedule(cpu);
>>   	put_cpu();
>>      

The atomic_xchg() does the trick.

-- 
error compiling committee.c: too many arguments to function