Re: [PATCH] pci: Refuse to hotplug PCI Devices when the Guest OS is not ready

On Fri, 23 Oct 2020 11:54:40 -0400 "Michael S. Tsirkin" <mst(a)redhat.com&gt; wrote: [...] [...] [...] [...] > > Rather than adding_device_allowed, something like "query slot" > might be helpful for debugging. That would help user figure out > e.g. why isn't device visible without any races. Would be new command useful tough? What we end up is broken guest (if I read commit message right) and a user who has no idea if device_add was successful or not. So what user should do in this case - wait till it explodes? - can user remove it or it would be stuck there forever? - poll slot before hotplug, manually? (if this is the case then failing device_add cleanly doesn't sound bad, it looks similar to another error we have "/* Check if hot-plug is disabled on the slot */" in pcie_cap_slot_pre_plug_cb) CCing libvirt, as it concerns not only QEMU. [...] [...] > > I think we want QEMU management interface to be reasonably > abstract and agnostic if possible. Pushing knowledge of hardware > detail to management will just lead to pain IMHO. > We supported device_add which practically never fails for years, For CPUs and RAM, device_add can fail so maybe management is also prepared to handle errors on PCI hotplug path.