Re: [PATCH] KVM: PPC: Book3S HV: Replace kvmppc_find_vcpu() with kvm_get_vcpu()

From: Thomas Huth <thuth@redhat.com>
To: Paul Mackerras <paulus@samba.org>
Cc: kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, agraf@suse.de,
	gleb@kernel.org, pbonzini@redhat.com, benh@kernel.crashing.org,
	mpe@ellerman.id.au
Subject: Re: [PATCH] KVM: PPC: Book3S HV: Replace kvmppc_find_vcpu() with kvm_get_vcpu()
Date: Fri, 8 May 2015 10:19:04 +0200	[thread overview]
Message-ID: <20150508101904.509c629a@thh440s> (raw)
In-Reply-To: <20150507230718.GA9410@iris.ozlabs.ibm.com>

On Fri, 8 May 2015 09:07:18 +1000
Paul Mackerras <paulus@samba.org> wrote:

> On Thu, May 07, 2015 at 05:49:39PM +0200, Thomas Huth wrote:
> > Both functions are doing the same thing - looking up the struct
> > kvm_vcpu pointer for a given vCPU ID.
> 
> No, kvm_get_vcpu(n) returns the n'th vcpu created, which is often
> but not necessarily the same as the vcpu with id n.  In fact, due to
> the way we do threading in HV KVM, it's quite common for userspace to
> create vcpus with ids 0, 8, 16, ..., which means that the n'th vcpu
> created does not have id n (except of course for vcpu 0).

Ah, ok, thanks a lot for the explanation ... I thought that KVM would
always put the VCPUs at the position of their ID into the array, but
looking at kvm_vm_ioctl_create_vcpu(), you're right, it's using the
amount of online CPUs as index instead:

	kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;

So never mind, and sorry for the nuisance!

 Thomas