5:21 a.m.
On Mon, Jul 29, 2024 at 08:29:42 -0700, Andrea Bolognani wrote:
On Mon, Jul 29, 2024 at 03:09:55PM GMT, Peter Krempa wrote:
> On Fri, Jul 26, 2024 at 07:47:57 -0700, Andrea Bolognani wrote:
> > For example, if the domain uses UEFI, local -> remote migration works
> > but when attempting to migrate back I get
> >
> > error: Requested operation is not valid: Setting different SELinux
> > label on /var/lib/libvirt/qemu/nvram/cirros-efi_VARS.fd which is
> > already in use
>
> So I've kind of forgot about this one. That's mainly as sharing this
> path is not actually needed for migration to work, as the contents are
> shared inside the migration stream automatically.
>
> Given though that users might want to share this one, so that they can
> start the VM withouth migration on a different host and also we allow
> setting the path to a more reasonable directory for sharing
> (users should not share /var/lib/libvirt/qemu/nvram/, or anything
> belonging to libvirt in the first place)
What's wrong with sharing this? It's just another type of storage
after all.
Well, in case of this specific directory it'd be okay. But we had users
wanting to share the XML directory too. In general I'd suggest user not
share anything that belongs to libvirt.
In worst case they'd share /var/lib/libvirt, which also contains stuff
that must not be shared, such as the master keys for qemu, dnsmasq data
files etc.
Similarly, sharing /var/lib/libvirt/qemu as whole is a bad idea.
And it's true that in some cases you might be able to
start the domain even with the NVRAM file being missing, but that's
not true in the general case.
Sure, this specific case will work. Arguably though you'd want to store
the nvrams in a location where you also have the VM images shared from,
so configuring specifically /var/li/libvirt/qemu/nvram to be shared
extra seems a bit weird.
Or are you saying that it's okay to share the directory where
NVRAM
files are stored, as long as it's not under /var/lib/libvirt?
As said, the nvram directory itself is okay, but it's under a directory
where other libvirt stuff is shared so users might misinterpret that.
Either way, we need to be able to handle the NVRAM image labels properly
regardless of where they are stored.
> > If I add TPM into the mix, local -> remote migration
fails with
> >
> > error: unable to lock
> > /var/lib/libvirt/swtpm/50601047-47f7-4f45-9b41-9112dfaa3539/tpm2/.lock
> > for metadata change: Resource temporarily unavailable
>
> This makes it look like the file is still locked, that might be
> indication of a different problem as libvirt will not keep the lock
> after updating the XATTRs.
I think there were some interesting timing issues with swtpm
attempting to lock the file while libvirt was trying to perform its
own metadata bookkeeping. I need to revisit that because honestly
I've forgotten almost everything about it.
IIRC libvirt uses a specific block to apply the lock on so that it's
locking it only for internal purposes. If 'swtpm' is locking the same
block it might cause problems.
> > One thing that concerns me is that the changes appear to
only be on
> > the destination side of the migration. So only the remote -> local
> > path is affected. But what about the local -> remote path?
>
> On the source side, at least for disks we simply nuke the
> metadata(xattrs). In fact we do that in most cases when stopping the VM,
> not only for migration.
>
> See qemuProcessStop(), in the loop which calls
> qemuBlockRemoveImageMetadata(). That call removes the metadata from all
> disk top level sources.
I see, thanks for the pointer.
> > If we assume remember_owner=1, then obviously the various XATTRs
> > related to remembering will be set on domain startup. What will take
> > care of collecting that garbage once local -> remote migration has
> > been performed? I think things might be fine based on the fact that
> > the XATTRs seems to be dropped for disks even in the current state,
> > but I thought I'd mention this specifically just in case.
>
> So apart from the code I've mentioned above which explicitly drops the
> xattrs, they are also not shared via NFS. Thus the remote side itself
> would not get them.
>
> If we'd leak them though it would mean that a re-start or incoming
> migration on the source of
> the migration would no longer work, so we need the same treatment for
> the other resources as well.
Right, that's exactly my concern. I have encountered at least one
case in which XATTRs not being cleaned up correctly on (failed?)
migration resulted in having to perform manual fixup tasks before the
domain would start again.
It'd be great to know when that happened.
> I'll post another version adding the missing treatment for
uefi, but I
> don't think I care enough about TPMs to go digging how that stuff is
> plumbed in or why it complains about locks being held which don't seem
> to be held by libvirt.
I will try to look into it myself on top of your updated patches. I'm
pretty sure TPM is a requirement for KubeVirt, so we definitely need
it to be handled correctly or the only recourse from their side will
be to set remember_owner=0 - and we've already established that we
don't want that to happen :)