Re: [libvirt] [Qemu-devel] Re: Libvirt debug API

On 04/23/2010 07:48 AM, Avi Kivity wrote: > On 04/22/2010 09:49 PM, Anthony Liguori wrote: >>> real API. Say, adding a device libvirt doesn't know about or >>> stopping the VM >>> while libvirt thinks it's still running or anything like that. >> Another problem is issuing Monitor commands that could confuse >> libvirt's >> >> We need to make libvirt and qemu smarter. >> >> We already face this problem today with multiple libvirt users. >> This is why sophisticated management mechanisms (like LDAP) have >> mechanisms to do transactions or at least a series of atomic >> operations. > > And people said qmp/json was overengineered... > > But seriously, transactions won't help anything. qemu maintains > state, and when you have two updaters touching a shared variable not > excepting each other to, things break, no matter how much locking > there is. Let's consider some concrete examples. I'm using libvirt and QMP and in QMP, I want to hot unplug a device. Today, I do this by listing the pci devices, and issuing a pci_del that takes a PCI address. This is intrinsically racy though because in the worst case scenario, in between when I enumerate pci devices and do the pci_del in QMP, in libvirt, I've done a pci_del and then a pci_add within libvirt of a completely different device. There are a few ways to solve this, the simplest being that we give devices unique ids that are never reused and instead of pci_del taking a pci bus address, it takes a device id. That would address this race.