7:54 a.m.
Hi all,
I just realized that libvirt default for directly attached interface is
vepa mode.
I discovered it now because virt-manager automatically enables bridge
mode, while cockpit-machines get the default vepa mode. This is
unfortunate because, being vepa switches very rare, it means that using
cockpit to configure directly attached interfaces causes guests to not
talk each other.
I have some questions:
- can libvirt default be changed/configured somehow (ie: to
automatically create bridge-mode directly attached interface when no
mode is specified)?
- how can I use virsh to discover machines with vepa-mode interfaces
(virsh domiflist <domain> does not return the interface mode)?
- can I change the interface type at runtime (virt-xml <domain> --edit
--network type=direct,source.mode=bridge works for inactive domains
only)?
Thanks.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti(a)assyoma.it - info(a)assyoma.it
GPG public key ID: FF5F32A8
12:32 p.m.
On 6/11/22 8:54 AM, Gionatan Danti wrote:
Hi all,
I just realized that libvirt default for directly attached interface is
vepa mode.
I discovered it now because virt-manager automatically enables bridge
mode, while cockpit-machines get the default vepa mode. This is
unfortunate because, being vepa switches very rare, it means that using
cockpit to configure directly attached interfaces causes guests to not
talk each other.
I have some questions:
- can libvirt default be changed/configured somehow (ie: to
automatically create bridge-mode directly attached interface when no
mode is specified)?
libvirt's defaults are defaults, and can't be changed with config.
libvirt also will never changed the hardcoded default, because that
could break existing installations.
I'm not sure why vepa was chosen as the default when macvtap support was
added to libvirt, but my best guess is that it was just the bias of the
person doing the work who assumed their usage would be the most common
(IIRC it was done by someone who was specifically wanting to support
connection to VEPA-capable switches, and since it was a new feature
nobody else had experience with / opinions about which mode would be
most common, so the reviewers just accepted this default)
- how can I use virsh to discover machines with vepa-mode interfaces
(virsh domiflist <domain> does not return the interface mode)?
I guess you'll need to do a "virsh dumpxml --inactive" for each guest
and parse it out of there.
- can I change the interface type at runtime (virt-xml <domain> --edit
--network type=direct,source.mode=bridge works for inactive domains only)?
No, I think the mode of the macvtap interface is set when the interface
is created, and can't be changed later (i.e. it's a limitation of
macvtap, not libvirt)
5:16 p.m.
Il 2022-06-11 19:32 Laine Stump ha scritto:
libvirt's defaults are defaults, and can't be changed with
config.
libvirt also will never changed the hardcoded default, because that
could break existing installations.
Thanks for the direct confirmation that default can not be changed with
config. I opened a cockpit-machine bug to ask for a change there:
https://bugzilla.redhat.com/show_bug.cgi?id=2095967
I'm not sure why vepa was chosen as the default when macvtap
support
was added to libvirt, but my best guess is that it was just the bias
of the person doing the work who assumed their usage would be the most
common (IIRC it was done by someone who was specifically wanting to
support connection to VEPA-capable switches, and since it was a new
feature nobody else had experience with / opinions about which mode
would be most common, so the reviewers just accepted this default)
Sound reasonable. Do you know if directly attached bridge-mode interface
is common and tested into production workload, or if "pure" bridge mode
is the way to go?
Full disclosure: I always used "pure" bridges, but I like the idea to
deny any guest->host traffic by design - hence my interest in
bridge-mode macvtap interfaces. Moreover, by the virtue of not requiring
a dedicated bridge interface, they can simplify the host network
configuration.
I guess you'll need to do a "virsh dumpxml --inactive"
for each guest
and parse it out of there.
It matches my results. However, virsh dumpxml is an expensive operation
- polkitd can easily burn >30% of a single core for 1s polling interval
with virsh dumpxml. I settled (for testing) with grepping "mode..vepa"
in /etc/libvirtd/qemu and it seems to work well (1s polling not even
register in top).
Does libvirt support calling some external script when a new virtual
machine is defined?
No, I think the mode of the macvtap interface is set when the
interface is created, and can't be changed later (i.e. it's a
limitation of macvtap, not libvirt)
Again, it matches my results (ip link showing "operation not supported"
when trying to change the mode at runtime). I hoped to miss something,
but it does not seem the case...
Thanks.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti(a)assyoma.it - info(a)assyoma.it
GPG public key ID: FF5F32A8
2:47 a.m.
On Sun, Jun 12, 2022 at 00:16:20 +0200, Gionatan Danti wrote:
Il 2022-06-11 19:32 Laine Stump ha scritto:
[...]
> I guess you'll need to do a "virsh dumpxml
--inactive" for each guest
> and parse it out of there.
It matches my results. However, virsh dumpxml is an expensive operation -
polkitd can easily burn >30% of a single core for 1s polling interval with
virsh dumpxml. I settled (for testing) with grepping "mode..vepa" in
/etc/libvirtd/qemu and it seems to work well (1s polling not even register
in top).
The unfortunate thing about using `virsh dumpxml $VM` as written is that
it opens a connection (which uses polkit to auth), gets the XML and then
closes the connection.
If you want it to be more optimized you can e.g. write a script using
python bindings for libvirt and simply keep the connection open ...
Does libvirt support calling some external script when a new virtual
machine
is defined?
... which additionally allows you to register 'domain lifecycle events'
[1] which give you a trigger when a new VM is defined.
https://www.libvirt.org/html/libvirt-libvirt-domain.html#virConnectDomain...
https://www.libvirt.org/html/libvirt-libvirt-domain.html#virDomainEventID
1:33 p.m.
Il 2022-06-13 09:47 Peter Krempa ha scritto:
If you want it to be more optimized you can e.g. write a script
using
python bindings for libvirt and simply keep the connection open ...
... which additionally allows you to register 'domain lifecycle events'
[1] which give you a trigger when a new VM is defined.
This is a good idea, indeed.
Thank you for suggesting!
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti(a)assyoma.it - info(a)assyoma.it
GPG public key ID: FF5F32A8
893
days inactive
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4 comments
3 participants
participants (3)
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Gionatan Danti -
Laine Stump -
Peter Krempa