linux-kernel - Re: [PATCH] KVM: don't sent IPI if the vcpu is not online

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Message-Id: <633F01D5-D6AD-40F3-8864-C3AE4D314E30@suse.de>
Date:	Mon, 6 Sep 2010 10:59:03 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Avi Kivity <avi@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: don't sent IPI if the vcpu is not online


On 06.09.2010, at 10:55, Avi Kivity wrote:

> On 09/06/2010 11:51 AM, Alexander Graf wrote:
>> 
>>> I think guest_mode makes sense for the other archs for reducing IPIs, so let's leave it common and recommend that they implement it.  Alex, if you're ever bored.
>> What does the bit do? Do we have documentation on it ;)? No seriously, what's the intent of the field?
>> 
> 
> It indicates that the vcpu is currently executing guest code.  Which in turn is important if you need to force it out of guest mode in order to inject an interrupt or flush the tlb.

Well, a vcpu is either offline in halt state or in guest mode, no? So we can conclude that guest_mode == !offline && !halt_state.

When in halt state, we are active on the wakeup waitqueue:

static void kvmppc_decrementer_func(unsigned long data)
{
        struct kvm_vcpu *vcpu = (struct kvm_vcpu *)data;

        kvmppc_core_queue_dec(vcpu);

        if (waitqueue_active(&vcpu->wq)) {
                wake_up_interruptible(&vcpu->wq);
                vcpu->stat.halt_wakeup++;
        }
}

Shouldn't that be enough information already?


Alex

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