On Tue, 24 Sep 2019 11:31:06 +0200 Christian Schoenebeck <qemu_...@crudebyte.com> wrote:
> On Montag, 23. September 2019 18:50:12 CEST Greg Kurz wrote: > > > > > If yes, and since that would mean I was the only person ever having > > > > > tested > > > > > the actual fix, shouldn't --multidevs=remap|forbid better be marked as > > > > > experimental (docs and runtime warning) for now? Maybe that would also > > > > > anticipate receiving feedback from people actually using it later on. > > > > > > > > Makes sense. I don't think it is worth having a runtime warning, > > > > but I'll turn remap to x-remap and amend the docs. > > > > > > Mwa, I would like to veto against your "x-remap" plan though. Keep in mind > > > I also have to send out a patch for libvirt for this fix. Even I would > > > not have read "x" to stand for "experimental". So I would definitely > > > favor a runtime warning instead of renaming that parameter. > > > > Hmmm... I don't see the point in adding a warning for a feature that > > is only active if the user explicitly asks for it. > > Because many people might be using this option without ever reading the docs, > e.g. because of being suggested by copy & paste tutorials or any stack*.com > forum. > > > And, anyway, this > > still is an experimental feature, right ? > > No, it is not a feature. It is still a fix. :) I cannot use 9p without this > fix at all, so it is not some optional "feature" for me. > I understand your need but this is still arguable. The 9p device has a limitation with cross-device setups. The actual bug is to silently cause inode number collisions in the guest. This is partly fixed by the "9p: Treat multiple devices on one export as an error" patch. Thinking again, it would even make sense to move "remap" from "9p: Added virtfs option 'multidevs=remap|forbid|warn'" to its own patch. We could then consider that the bug is fully fixed with "multidevs=forbid|warn". Then comes the "remap" feature which is expected to lift the limitation with cross-device setups, with a "not yet determined" performance cost and light reviewing of the code. > x-remap would just make things unnecessarily more complicated without adding > anything useful. > Not really. This gives a crucial information to the user about the level of confidence we have in this feature. > A warning/info log could be used to motivate people providing feedback and > make them clearly aware about their current version still being an > experimental fix in their specific qemu version. That warning/info is just a > one line change that can easily be dropped at some point in future after this > qid fix proofed to be reliable (i.e. from users' feedback and test cases). > The overwhelming majority of feedbacks I had on 9p the last few years are CVEs. Antonios and you are the only users who ever seemed to care for cross-device setups. So I don't expect much feedback on that area and I don't buy the "motivate people" argument, especially since "remap" won't be the default. > > Not sure it is time to have > > libvirt to support it yet. > > Most people are using libvirt xml configs instead of calling qemu directly > with command line options. So of course I will suggest a libvirt patch as > soon > as this patch set is pulled on qemu side. > Yes and before a feature has a chance to be officially supported in libvirt, people usually rely on the <qemu:commandline> domain XML tag to pass extra arguments to QEMU. https://blog.vmsplice.net/2011/04/how-to-pass-qemu-command-line-options.html With the current fsdev implementation, we can only pass properties to the -fsdev command line option. So this would require to not use the <filesystem type='mount'> XML tag and manually _open-code_ the needed QEMU arguments: <qemu:commandline> <qemu:arg value='-fsdev'/> <qemu:arg value='local,id=fsdev0,path=/var/tmp/virtfs,security_model=passthrough,multidevs=remap'/> <qemu:arg value='-device'/> <qemu:arg value='virtio-9p,id=virtio-9p0,mount_tag=host,fsdev=fsdev0'/> </qemu:commandline> And if fsdev is converted to be a proper QEMU device, it would as easy as: <qemu:commandline> <qemu:arg value='-set'/> <qemu:arg value='device.fsdev0.multidevs=remap'/> </qemu:commandline> This is unrelated but it would also allow to drop a lot of code in fsdev that mimics what qdev would give us for free. :) > > Also, I strongly recommend you try out "virtio-fs" which is going to be soon the production grade way of sharing files between host and guest. https://www.mail-archive.com/libvir-list@redhat.com/msg182457.html Cheers, -- Greg
