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Re: [libvirt] [PATCH 0/8] Hostdev-hybrid patches



On Tue, Sep 11, 2012 at 03:07:25PM -0400, Laine Stump wrote:
> On 09/07/2012 12:12 PM, Shradha Shah wrote:
> > This patch series adds the support for interface-type="hostdev-hybrid" and
> > forward mode="hostdev-hybrid".
> >
> > The hostdev-hybrid mode makes migration possible along with PCI-passthrough.
> > I had posted a RFC on the hostdev-hybrid methodology earlier on the libvirt
> > mailing list.
> >
> > The RFC can be found here:
> > https://www.redhat.com/archives/libvir-list/2012-February/msg00309.html
> 
> Before anything else, let me outline what I *think* happens with a
> hostdev-hybrid device entry, and you can tell me how far off I am :-):
> 
> * Any hostdev-hybrid interface definition results in 2 PCI devices being
> added to the guest:
> 
>    a) a PCI passthrough of an SR-IOV VF (done essentially the same as
>       <interface type='hostdev'>
>    b) a virtio-net device which is connected via macvtap "bridge" mode
>       (? is that always the case) to the PF of the VF in (a)
> 
> * Both of these devices are assigned the same MAC address.
> 
> * Each of these occupies one PCI address on the guest, so a total of 2
> PCI addresses is needed for each hostdev-hybrid "device". (The
> redundancy in this statement is to be sure that I'm right, as that's an
> important point :-)
> 
> * On the guest, these two network devices with matching MAC addresses
> are put together into a bond interface, with an extra driver that causes
> the bond to prefer the pci-passthrough device when it is present. So,
> under normal circumstances *all* traffic goes through the
> pci-passthrough device.
> 
> * At migration time, since guests with attached pci-passthrough devices
> can't be migrated, the pci-passthrough device (which is found by
> searching the hostdev array for items with the "ephemeral" flag set) is
> detached. This reduces the bond interface on the guest to only having
> the virtio-net device, so traffic now passes through that device - it's
> slower, but connectivity is maintained.
> 
> * on the destination, a new VF is found, setup with proper MAC address,
> VLAN, and 802.1QbX port info. A virtio-net device attached to the PF
> associated with this VF (via macvtap bridge mode) is also setup. The
> qemu commandline includes an entry for both of these devices. (Question:
> Is it the virtio-net device that uses the guest PCI address given in the
> <interface> device info?) (Question: actually, I guess the
> pci-passthrough device won't be attached until after the guest actually
> starts running on the destination host, correct?)
> 
> * When migration is finished, the guest is shut down on the source and
> started up on the destination, leaving the new instance of the guest
> temporarily with just a single (virtio-net) device in the bond.
> 
> * Finally, the pci-passthrough of the VF is attached to the guest, and
> the guest's bond interface resumes preferring this device, thus
> restoring full speed networking.
> 
> Is that all correct?
> 
> If so, one issue I have is that one of the devices (the
> pci-passthrough?) doesn't have its guest-side PCI address visible
> anywhere in the guest's XML, does it? This is problematic, because
> management applications (and libvirt itself) expect to be able to scan
> the list of devices to learn what PCI slots are occupied on the guest,
> and where they can add new devices.

If that description is correct, then I have to wonder why we need to
add all this code for a new "hybrid" device type. It seems to me like
we can do all this already simply by listing one virtio device and one
hostdev device in the guest XML. All that's required is to add support
for the 'ephemeral' against hostdevs, so they are automagically
unplugged. Technically we don't even need that, since a mgmt app can
already just use regular hotunplug APIs before issuing the migrate
API calls.  These patches seem to add alot of complexity for mere
syntactic sugar over existing capabilities.


Daniel
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