On Fri, Aug 16, 2024 at 05:00:32PM +0100, Daniel P. Berrangé wrote:
On Fri, Aug 16, 2024 at 11:34:10AM -0400, Peter Xu wrote:
On Fri, Aug 16, 2024 at 04:16:50PM +0100, Daniel P. Berrangé wrote:
On Fri, Aug 16, 2024 at 11:06:10AM -0400, Peter Xu wrote:
On Thu, Aug 15, 2024 at 04:55:20PM -0400, Steven Sistare wrote:
On 8/13/2024 3:46 PM, Peter Xu wrote:
On Tue, Aug 06, 2024 at 04:56:18PM -0400, Steven Sistare wrote:
The flipside, however, is that localhost migration via 2 separate QEMU
processes has issues where both QEMUs want to be opening the very same
file, and only 1 of them can ever have them open.
I thought we used to have similar issue with block devices, but I assume
it's solved for years (and whoever owns it will take proper file lock,
IIRC, and QEMU migration should properly serialize the time window on who's
going to take the file lock).
Maybe this is about something else?
I don't have an example where this fails.
I can cause "Failed to get "write" lock" errors if two qemu instances open
the same block device, but the error is suppressed if you add the -incoming
argument, due to this code:
blk_attach_dev()
if (runstate_check(RUN_STATE_INMIGRATE))
blk->disable_perm = true;
Yep, this one is pretty much expected.
Indeed, and "files" includes unix domain sockets.
More on this -- the second qemu to bind a unix domain socket for listening
wins, and the first qemu loses it (because second qemu unlinks and recreates
the socket path before binding on the assumption that it is stale).
One must use a different name for the socket for second qemu, and clients
that wish to connect must be aware of the new port.
Network ports also conflict.
cpr-exec avoids such problems, and is one of the advantages of the method that
I forgot to promote.
I was thinking that's fine, as the host ports should be the backend of the
VM ports only anyway so they don't need to be identical on both sides?
IOW, my understanding is it's the guest IP/ports/... which should still be
stable across migrations, where the host ports can be different as long as
the host ports can forward guest port messages correctly?
Yes, one must use a different host port number for the second qemu, and clients
that wish to connect must be aware of the new port.
That is my point -- cpr-transfer requires fiddling with such things.
cpr-exec does not.
Right, and my understanding is all these facilities are already there, so
no new code should be needed on reconnect issues if to support cpr-transfer
in Libvirt or similar management layers that supports migrations.
Note Libvirt explicitly blocks localhost migration today because
solving all these clashing resource problems is a huge can of worms
and it can't be made invisible to the user of libvirt in any practical
way.
Ahhh, OK. I'm pretty surprised by this, as I thought at least kubevirt
supported local migration somehow on top of libvirt.
Since kubevirt runs inside a container, "localhost" migration
is effectively migrating between 2 completely separate OS installs
(containers), that happen to be on the same physical host. IOW, it
is a cross-host migration from Libvirt & QEMU's POV, and there are
no clashing resources to worry about.
OK, makes sense.
Then do you think it's possible to support cpr-transfer in that scenario
from Libvirt POV?
Does it mean that cpr-transfer is a no-go in this case at least for Libvirt
to consume it (as cpr-* is only for local host migrations so far)? Even if
all the rest issues we're discussing with cpr-exec, is that the only way to
go for Libvirt, then?
cpr-exec is certainly appealing from the POV of avoiding the clashing
resources problem in libvirt.
It has own issues though, because libvirt runs all QEMU processes with
seccomp filters that block 'execve', as we consider QEMU to be untrustworthy
and thus don't want to allow it to exec anything !
I don't know which is the lesser evil from libvirt's POV.
Personally I see security controls as an overriding requirement for
everything.
One thing I am aware of is cpr-exec is not the only one who might start to
use exec() in QEMU. TDX fundamentally will need to create another key VM to
deliver the keys and the plan seems to be using exec() too. However in
that case per my understanding the exec() is optional - the key VM can also
be created by Libvirt.
IOW, it looks like we can still stick with execve() being blocked yet so
far except cpr-exec().
Hmm, this makes the decision harder to make. We need to figure out a way
on knowing how to consume this feature for at least open source virt
stack.. So far it looks like it's only possible (if we take seccomp high
priority) we use cpr-transfer but only in a container.