linux-kernel - Re: [PATCH 4/5] KVM: cpu hotplug support

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Message-ID: <20070131085039.GA17141@elte.hu>
Date:	Wed, 31 Jan 2007 09:50:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...l.org>
Cc:	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] KVM: cpu hotplug support


* Andrew Morton <akpm@...l.org> wrote:

> On Tue, 30 Jan 2007 14:56:16 -0000
> Avi Kivity <avi@...ranet.com> wrote:
> 
> > +static void decache_vcpus_on_cpu(int cpu)
> > +{
> > +	struct kvm *vm;
> > +	struct kvm_vcpu *vcpu;
> > +	int i;
> > +
> > +	spin_lock(&kvm_lock);
> > +	list_for_each_entry(vm, &vm_list, vm_list)
> > +		for (i = 0; i < KVM_MAX_VCPUS; ++i) {
> > +			vcpu = &vm->vcpus[i];
> > +			/*
> > +			 * If the vcpu is locked, then it is running on some
> > +			 * other cpu and therefore it is not cached on the
> > +			 * cpu in question.
> > +			 *
> > +			 * If it's not locked, check the last cpu it executed
> > +			 * on.
> > +			 */
> > +			if (mutex_trylock(&vcpu->mutex)) {
> > +				if (vcpu->cpu == cpu) {
> > +					kvm_arch_ops->vcpu_decache(vcpu);
> > +					vcpu->cpu = -1;
> > +				}
> > +				mutex_unlock(&vcpu->mutex);
> > +			}
> > +		}
> > +	spin_unlock(&kvm_lock);
> > +}
> 
> The trylock is unpleasing.  Perhaps kvm_lock should be a mutex or 
> something?

this is a special case. The vcpu->mutex acts as a 'this vcpu is running 
right now' flag as well - hence the trylock signals: is it running right 
now or not - if it's not running we do not have to 'decache' it. But i 
agree and i already suggested to Avi to change kvm_lock to be a mutex - 
but this wont change the trylock.

	Ingo
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