Re: [PATCH 3/3] kvm common: verify that cpu slot is available when creating new vcpu

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Feb 05, 2009 at 05:05:01PM +0100, Carsten Otte wrote:
> KVM common code should'nt try to create the same virtual cpu twice.
> In case of s390, it crashes badly in kvm_arch_vcpu_create.
> 
> Reported-by: Mijo Safradin <mijo@linux.vnet.ibm.com>
> Signed-off-by: Carsten Otte <cotte@de.ibm.com>
> ---
>  virt/kvm/kvm_main.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> Index: kvm/virt/kvm/kvm_main.c
> ===================================================================
> --- kvm.orig/virt/kvm/kvm_main.c
> +++ kvm/virt/kvm/kvm_main.c
> @@ -1605,6 +1605,9 @@ static int kvm_vm_ioctl_create_vcpu(stru
>  	if (!valid_vcpu(n))
>  		return -EINVAL;
>  
> +	if (kvm->vcpus[i])
> +		return -EEXIST;
> +
>  	vcpu = kvm_arch_vcpu_create(kvm, n);
>  	if (IS_ERR(vcpu))
>  		return PTR_ERR(vcpu);

Its confusing that there is the exact same check below, with kvm->lock
held, and that both are needed since assignment happens under the lock.

Can you also make it straightforward while fixing the bug please.

Probably just hold it all the way through kvm_vm_ioctl_create_vcpu? Or
is that not possible?