Re: Questions regarding potential resize implementation in libvirt
On Thu, Mar 20, 2025 at 08:56:04AM +0100, Maximilian Immanuel Brandtner wrote:
I've been working on an RFC patch-set to implement resizing for
consoles in QEMU. Now that the patch-set is in review I've turned my
attention to bringing this feature to libvirt.
Can you point to the QEMU patchset URL ?
With the QEMU patch-set there are two ways to resize a pty chardev.
1. send a TIOCSWINSZ ioctl
2. send a QMP msg
The first approach doesn't work under Libvirt. Pty chardevs are proxied
over a FIFO meaning the ioctl wouldn't get to libvirt. Furthermore,
this approach is incompatible with remote management which libvirt
seems to go to great lengths to support.
So we have the flow
virsh console (or equiv mgmt equiv)
-> virStream
-> libvirtd/virtqemud
-> virStream
-> PTY
-> QEMU chardev
-> guest serial port / paravirt console
NB I've always wanted to extend our console stream handling to support
UNIX socket / TCP socket chardev instead of the PTY, as the connection
semantics are so much saner to deal with.
Sending a QMP message is a QEMU specific feature. It seems to me
that
adding a new handle (something like domainResizeConsole) would have to
be added to the virHypervisorDriver struct. It would be really nifty if
the resize handle could be implemented in the virStream, however it
seems that all the virStreamDriver handlers are common code (aka shared
across all hypervisors). From what I've seen it seems that the only way
to switch from common code to hypervisor specific code would be over
the virHypervisorDriver. As it stands it seems to me that the best
course of action would be adding a new handle to the
virHypervisorDriver, but I'm unsure whether that's really the right
call.
How is the window resize supposed to be passed to the QEMU chardev
backend for the different chardev backend types ?
TLDR I have the following 2 questions:
- Should the resize handle be implemented in the virHypervisorDriver
directly or in the virStreamDriver?
Unclear currently. I did think about this a long while back and tried
to hack something into virStream, but never completed it and it was
not as simple as it first seemed
- Is there a way to get the QMP alias of a chardev from a virStream?
No, currently the stream only knows about the underlying host OS channel,
nothing about the hypervisor
With regards,
Daniel
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