[libvirt] Exposing mem-path in domain XML

[Daniel P. Berrange]

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.

Regards,
Daniel
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