[libvirt] Question about pci-passthrough

[Laine Stump]

On 07/26/2012 11:53 AM, Shradha Shah wrote:
> Hello All,
>
> I had a question about pci-passthrough using hostdev mode in Libvirt.
>
> When I assign hostdev->parent.type = VIR_DOMAIN_DEVICE_NET, the hostdev is passed into the guest and acts as a network device.

Actually, aside from libvirt setting the MAC address prior to assigning
the device to the guest, all the rest of the handling is identical to
that of a PCI device.

>
> When I assign hostdev->parent.type = VIR_DOMAIN_DEVICE_NONE, the hostdev is passed into the guest and acts as a PCI Host device.
>
> Please can someone point me to the part of code in libvirt where this decision is made based on hostdev->parent.type?
>

I'm curious why you're setting those yourself. that's really something
internal that should only be set when parsing a device defined as
<interface type='hostdev'> - this ends up creating entries for both
types of devices, a virDomainNetDef and a virDomainHostdevDef, that are
tightly intertwined.

If you attempt to just allocate your own virDomainHostdevDef and set
hostdev->parent.type = VIR_DOMAIN_DEVICE_NET, you will have a "very bad
time" (tm). The code assumes that any HostdevDef that has a parent-type
!= NONE is not a standalone object, but is one that resides within a
higher level object (see the definition of _virDomainNetDef, in
particular the  "virDomainHostdevDef def" that is defined within it).
The *only way* to setup one of these objects that will work properly is
to create a virDomainNetDef object (e.g. call it "net"), then set
net->type = VIR_DOMAIN_NET_TYPE_HOSTDEV, and initialize
net->data.hostdev.dev with all the hostdev info, including pointing its
"parent back to the original "net".
Likewise,  the virDomainNetDef that you create *must* be placed on the
domain's list of network devices and &net->data.hostdev.def *must* be
placed on the domain's list of hostdevs. All of this is handled for you
by virDomainNetDefParse (look for the VIR_DOMAIN_NET_TYPE_HOSTDEV case).

As to where the decision is made about how to treat the device - for
starters, <interface type='hostdev'> devices are never attached by
calling the function to do a hostdev attach
(qemuDomainAttachHostDevice)directly  - they are attached by calling the
function that does an attach of a net device (qemuDomainAttachNetDevice)
- that function calls networkAllocateActualDevice if necessary (in case
it's a type='hostdev' hiding behind a type='network') and then if the
"actualType" is hostdev, calls qemuDomainAttachHostDevice to do the rest
of the work. qemuDomainAttachHostDevice will call down a couple of
levels to qemuPrepareHostdevPCIDevices, which will check for
hostdev->parent.type == VIR_DOMAIN_DEVICE_NET, and if so will call
qemuDomainHostdevNetConfigReplace, which calls the functions that do the
netdev-specific setup (i.e. setting the MAC address, although I plan to
also add support for setting the VLAN tag).

So, a more organized version of this:

1) if you want a hybrid netdev/hostdev, start by creating a
virDomainNetDef and filling in the hostdev part, not vice versa, and
make sure the corresponding parts are on their respective lists for the
domain. (hostdevs and nets)

2) to attach/detach any of these devices, call the *net* version of the
attach/detach functions, not the hostdev version.

3) The place that checks if a particular device is a network-type of
hostdev, is qemuPrepareHostdevPCIDevices - look for
"hostdev->parent.type == VIR_DOMAIN_DEVICE_NET" in that function and
follow the call chain down from there to where the action is.