[libvirt] [RFC PATCH] hostdev: add support for "managed='detach'"

[Alex Williamson]

On Thu, 17 Mar 2016 17:32:08 +0000
"Daniel P. Berrange" <berrange at redhat.com> wrote:

> On Tue, Mar 15, 2016 at 02:21:35PM -0400, Laine Stump wrote:
> > On 03/15/2016 01:00 PM, Daniel P. Berrange wrote:  
> > >On Mon, Mar 14, 2016 at 03:41:48PM -0400, Laine Stump wrote:  
> > >>Suggested by Alex Williamson.
> > >>
> > >>If you plan to assign a GPU to a virtual machine, but that GPU happens
> > >>to be the host system console, you likely want it to start out using
> > >>the host driver (so that boot messages/etc will be displayed), then
> > >>later have the host driver replaced with vfio-pci for assignment to
> > >>the virtual machine.
> > >>
> > >>However, in at least some cases (e.g. Intel i915) once the device has
> > >>been detached from the host driver and attached to vfio-pci, attempts
> > >>to reattach to the host driver only lead to "grief" (ask Alex for
> > >>details). This means that simply using "managed='yes'" in libvirt
> > >>won't work.
> > >>
> > >>And if you set "managed='no'" in libvirt then either you have to
> > >>manually run virsh nodedev-detach prior to the first start of the
> > >>guest, or you have to have a management application intelligent enough
> > >>to know that it should detach from the host driver, but never reattach
> > >>to it.
> > >>
> > >>This patch makes it simple/automatic to deal with such a case - it
> > >>adds a third "managed" mode for assigned PCI devices, called
> > >>"detach". It will detach ("unbind" in driver parlance) the device from
> > >>the host driver prior to assigning it to the guest, but when the guest
> > >>is finished with the device, will leave it bound to vfio-pci. This
> > >>allows re-using the device for another guest, without requiring
> > >>initial out-of-band intervention to unbind the host driver.  
> > >You say that managed=yes causes pain upon re-attachment and that
> > >apps should use managed=detach to avoid it, but how do management
> > >apps know which devices are going to cause pain ? Libvirt isn't
> > >providing any info on whether a particular device id needs to
> > >use managed=yes vs managed=detach, and we don't want to be asking
> > >the user to choose between modes in openstack/ovirt IMHO. I think
> > >thats a fundamental problem with inventing a new value for managed
> > >here.  
> > 
> > My suspicion is that in many/most cases users don't actually need for the
> > device to be re-bound to the host driver after the guest is finished with
> > it, because they're only going to use the device to assign to a different
> > guest anyway. But because managed='yes' is what's supplied and is the
> > easiest way to get it setup for assignment to a guest, that's what they use.
> > 
> > As a matter of fact, all this extra churn of changing the driver back and
> > forth for devices that are only actually used when they're bound to vfio-pci
> > just wastes time, and makes it more likely that libvirt and its users will
> > reveal and get caught up in the effects of some strange kernel driver
> > loading/unloading bug (there was recently a bug reported like this;
> > unfortunately the BZ record had customer info in it, so it's not publicly
> > accessible :-( )
> > 
> > So beyond making this behavior available only when absolutely necessary, I
> > think it is useful in other cases, at the user's discretion (and as I
> > implied above, I think that if they understood the function and the
> > tradeoffs, most people would choose to use managed='detach' rather than
> > managed='yes')  
> 
> IIUC, in managed=yes mode we explicitly track whether the device was
> originally attached to a host device driver. ie we only re-attach
> the device to the host when guest shuts down, if it was attached to
> the host at guest startup.
> 
> We already have a virNodeDeviceDetach() API that can be used to
> detach a device from the host driver explicitly.
> 
> So applications can in fact already achieve what you describe in
> terms of managed=detach, by simply calling virNodeDeviceDetach()
> prior to starting the guest with cold plugged PCI devices / hotplugging
> the PCI device.
> 
> IOW, even if we think applications should be using managed=detach,
> they can already do so via existing libvirt APIs.

Agreed, that was never in doubt, but now you've required a management
step before the VM can be started.  We're basically requiring users to
write their own scripting or modify kernel commandlines simply to avoid
libvirt trying to rebind a device at shutdown.  We're leaving it as the
users' problem how to handle autostart VMs that prefer this behavior.
It can already be done, the question is whether it's worthwhile to make
this easier path to do it.  Thanks,

Alex