[libvirt] A question about scsi controller index

Hi, all I define a VM with a virtio scsi disk and I specify a controller index. <disk type='file' device='disk'> <driver name='qemu' type='raw' cache='none' io='native'/> <source file='/mnt/sdb/wangrui/img/sles10_sp3_32_2U-5'/> <target dev='sdb' bus='scsi'/> <address type='drive' controller='10' bus='0' target='0' unit='1'/> </disk> Controllers 0-10 are created(in virDomainDefAddDiskControllersForType()) although I have only one scsi disk. Are the controllers 0-9 useful for some reason? If the user specifies controller='n', it will cost extra time and memory for creating controllers from 0 to n-1. It becomes worse if n is a very large value (Usually it's the user's wrong configuration). I think if controllers from 0 to n-1 are kept for other use , we should better to check whether n is meaningless(A very large number is meaningless). BTW, I modify this function. Only the specified index controller will be created. It seems to work fine.