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Re: [libvirt] Question about pci-passthrough



Hello Laine,

Thanks for your help. Please find my comments inline.

On 07/27/2012 08:41 PM, Laine Stump wrote:
> 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.
I am working on HOSTDEV_HYBRID patches. I am following the exact same steps
as for HOSTDEV mode. 
So in the Hybrid mode I need a net device in the guest along with the VF,
the VF needs to be pushed into the guest as a PCI device.

> 
> 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).

Agreed
> 
> 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).
Agreed

> 
> 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)
I have followed this step, Created the virDomainNetDef and filled in the 
hostdev part but I had to assign hostdev->parent.type = VIR_DOMAIN_DEVICE_NONE
as I wanted to push the VF as a PCI device into the guest.
> 
> 2) to attach/detach any of these devices, call the *net* version of the
> attach/detach functions, not the hostdev version.
I have correctly followed this step as well.
> 
> 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.
The only problem now is, since hostdev->parent.type = VIR_DOMAIN_DEVICE_NONE
I cannot assign the VF a mac address. 
If I change hostdev->parent.type = VIR_DOMAIN_DEVICE_NET, I am able to assign 
VF a mac address but the VF is pushed into the guest as a net device not a 
PCI device.
> 

So I am able to push the VF into the guest as a PCI device along with a virio
net device but I cannot resolve the problem of assigning a MAC address to the 
VF. This is the only issue remaining in the HOSTDEV_HYBRID patches.

Many Thanks,
Regards,
Shradha Shah


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