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

[Daniel P. Berrange]

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.

Can you provide more details about the problems with detaching ?

Is this inherant to all VGA cards, or is it specific to the Intel
i915, or specific to a kernel version or something else ?

I feel like this is something where libvirt should "do the right
thing", since that's really what managed=yes is all about.

eg, if we have managed=yes and we see an i915, we should
automatically skip re-attach for that device.

Regards,
Daniel
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