From: Avi Kivity <avi@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: Get rid of KVM_REQ_KICK
Date: Tue, 04 May 2010 19:42:45 +0300 [thread overview]
Message-ID: <4BE04E85.8020608@redhat.com> (raw)
In-Reply-To: <20100504163150.GC20497@amt.cnet>
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
next prev parent reply other threads:[~2010-05-04 16:42 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-03 14:19 [PATCH] KVM: Get rid of KVM_REQ_KICK Avi Kivity
2010-05-04 16:31 ` Marcelo Tosatti
2010-05-04 16:42 ` Avi Kivity [this message]
2010-05-04 16:49 ` Avi Kivity
2010-05-06 19:56 ` Marcelo Tosatti
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