[ovirt-devel] [vdsm] Infrastructure design for node (host) devices

[Saggi Mizrahi]

----- Original Message -----
> From: "Martin Polednik" <mpoledni at redhat.com>
> To: devel at ovirt.org
> Sent: Tuesday, June 24, 2014 1:26:17 PM
> Subject: [ovirt-devel] [vdsm] Infrastructure design for node (host) devices
> 
> Hello,
> 
> I'm actively working on getting host device passthrough (pci, usb and scsi)
> exposed in VDSM, but I've encountered growing complexity of this feature.
> 
> The devices are currently created in the same manner as virtual devices and
> their reporting is done via hostDevices list in getCaps. As I implemented
> usb and scsi devices, the size of this list grew almost twice - and that is
> on a laptop.
There should be a separate verb with ability to filter by type.
> 
> Similar problem is with the devices themselves, they are closely tied to host
> and currently, engine would have to keep their mapping to VMs, reattach back
> loose devices and handle all of this in case of migration.
Migration sound very complicated, especially at the phase where the VM actually
starts running on the target host. The hardware state is completely different
but the guest OS wouldn't have any idea that happened.
So detaching before migration and than reattaching on the destination is a must
but that could cause issues in the guest. I'd imaging that this would be an issue
when hibernating on one host and waking up on another.
> 
> I would like to hear your opinion on building something like host device pool
> in VDSM. The pool would be populated and periodically updated (to handle
> hot(un)plugs) and VMs/engine could query it for free/assigned/possibly
> problematic
> devices (which could be reattached by the pool). This has added benefit of
> requiring fewer libvirt calls, but a bit more complexity and possibly one
> thread.
> The persistence of the pool on VDSM restart could be kept in config or
> constructed
> from XML.
I'd much rather VDSM not cache state unless this is absolutely necessary.
This sounds like something that doesn't need to be queried every 3 seconds
so it's best if we just get to ask libvirt.

I do wonder how that kind of thing can be configured in the VM creation
phase as you would sometimes want to just specify a type of device and
sometimes specify a specific one. Also, I'd assume there will be a
fallback policy stating if the VM should run if said resource is unavailable.
> 
> I'd need new API verbs to allow engine to communicate with the pool,
> possibly leaving caps as they are and engine could detect the presence of
> newer
> vdsm by presence of these API verbs.
Again, I think that getting a list of devices filterable by kind\type might
be best than a real pool. We might want to return if a device is in use
(could also be in use by the host operating system and not just VMs)
> The vmCreate call would remain almost
> the
> same, only with the addition of new device for VMs (where the detach and
> tracking
> routine would be communicated with the pool).
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