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

On 07/28/2017 10:59 AM, Daniel P. Berrange wrote: > 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. This might get you answer: https://bugzilla.redhat.com/show_bug.cgi?id=1461214#c4 So the way I understand it is that they will create the files, and provide us with paths. So luckily, we don't have to make up the paths on our own.