Re: [libvirt] Exposing mem-path in domain XML

On Fri, Jul 28, 2017 at 10:45:21AM +0200, Michal Privoznik wrote: > On 07/27/2017 03:50 PM, Daniel P. Berrange wrote: >> On Thu, Jul 27, 2017 at 02:11:25PM +0200, Michal Privoznik wrote: >>> Dear list, >>> >>> there is the following bug [1] which I'm not quite sure how to grasp. So >>> there is this application/infrastructure called Kove [2] that allows you >>> to have memory for your application stored on a distant host in network >>> and basically fetch needed region on pagefault. Now imagine that >>> somebody wants to use it for backing up domain memory. However, the way >>> that the tool works is it has some kernel module and then some userland >>> binary that is fed with the path of the mmaped file. I don't know all >>> the details, but the point is, in order to let users use this we need to >>> expose the paths for mem-path for the guest memory. I know we did not >>> want to do this in the past, but now it looks like we don't have a way >>> around it, do we? >> >> We don't want to expose the concept of paths in the XML because this is >> a linux specific way to configure hugepages / shared memory. So we hide >> the particular path used in the internal impl of the QEMU driver, and >> or via the qemu.conf global config file. I don't really want to change >> that approach, particularly if the only reason is to integrate with a >> closed source binary like Kove. > > Yep, I agree with that. However, if you read the discussion in the > linked bug you'll find that they need to know what file in the > memory_backing_dir (from qemu.conf) corresponds to which domain. The > reported suggested using UUID based filenames, which I fear is not > enough because one can have multiple <memory type='dimm'/> -s configured > for their domain. But I guess we could go with: > > ${memory_backing_dir}/${domName} for generic memory > ${memory_backing_dir}/${domName}_N for Nth <memory/> This feels like it is going to lead to hell when you add in memory hotplug/unplug, with inevitable races. > BTW: IIUC they want predictable names because they need to create the > files before spawning qemu so that they are picked by qemu instead of > using temporary names. I would like to know why they even need to associate particular memory files with particular QEMU processes. eg if they're just exposing a new type of tmpfs filesystem from the kernel why does it matter what each file is used for.