Re: [PATCH v2 05/13] conf: Introduce virtio-mem <memory/> model

On Mon, Feb 22, 2021 at 16:57:11 +0100, David Hildenbrand wrote: > On 22.02.21 16:46, Michal Privoznik wrote: >> On 2/18/21 4:00 PM, David Hildenbrand wrote: >>> On 18.02.21 14:31, Michal Privoznik wrote: [...] > In QEMU, we could make "info balloon" etc. also include virtio-mem provided > memory. > > The main reason I did not do so initially is that > 1) It's racy when reading/writing the balloon size. The QEMU interface > is broken as we don't get/set the size of the balloon size but > instead the logical VM size. While someone sets the logical VM size > via virtio-balloon, the logical VM size might change due to virtio- > mem guest activity. > 2) I don't want people to be tempted to use both at the same > time. > > Maybe the right think to do is make "info balloon" report the current > logical VM size. If there are races, bad luck - better not use both things > at the same time. We specifically don't call info balloon or any equivalent nowadays if qemu supports the balloon event and we should keep it like that at least in terms of the active XML. The bulk stats functions we have already call the monitor so it's okay to do it there. Both virtio-mem and balloon do have events so we should be able to update any required definition bits without an explicit call to the balloon info apart from the stats code and the refresh on reconnect.