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Re: [libvirt] [PATCH 0/5] Interface pools and passthrough mode



On 11/30/2011 01:29 AM, Laine Stump wrote:
> On 11/29/2011 02:53 PM, Daniel P. Berrange wrote:
>> On Tue, Nov 29, 2011 at 03:46:13PM +0000, Shradha Shah wrote:
>>> Interface Pools and Passthrough mode:
>>>
>>> Current Method:
>>> The passthrough mode uses a macvtap a direct connection to connect each guest to the network. The physical interface to be used is picked from among those listed in<interface>  sub elements of the<forward>  element.
>>>
>>> The current specification for<forward>  extends to allow 0 or more<interface>  sub-elements:
>>> Example:
>>> <forward mode='passthrough' dev='eth10'/>
>>> <interface dev='eth10'/>
>>> <interface dev='eth12'/>
>>> <interface dev='eth18'/>
>>> <interface dev='eth20'/>
>>> </forward>
>>>
>>> However with an ethernet card with 64 VF's or more, the above method gets tedious on the system.
>>>
>>> On the other hand, just parameterizing a string (eth%d) is inadequate, eg, when there are multiple non-contiguous ranges.
> 
> 
> Heh. You've gone through some of the same thought process I went through when I wrote the original code :-)
> 
> 
>>>
>>> Proposed Method:
>>> The 5 patches provided along with this introductory e-mail
>>>
>>> i) Introduce a structure to store the state of all the virtual functions attached to each physical function
>>> ii) Find a free virtual function given the physical function.
>>>
>>> The forward specification will hence only mention the physical function as the interface sub element:
>>> Example:
>>> <forward mode='passthrough' dev='eth2'/>
>>> <interface dev='eth2'/>
>>> </forward>
>> I can see what you mean about it being tedious to construct the config with
>> all 64 (or more) VF's listed, but this proposed change has a couple of issues
>>
>> First of all, the change you describe would be a semantic change in the
>> network XML, which would break compatibility with previous releases of
>> libvirt. Since we consider XML to be poart of our long term ABI stability
>> guarantee I don't think we can do the change.
>>
>> Ignoring the ABI issue, I'm concerned that as we get PFs with an increasingly
>> large number of VFs, we may well *not* want to associate all VFs with a single
>> virtual network definition. eg, we might wna to put 32 VFs in one network and
>> 32 VFs in another network.  Or if we have 2 PFs, we might want to interleave
>> VFs from several PFs across virtual networks. If all we can do is list the
>> PF in the XML, we loose significant flexibility in how VFs are assigned.
> 
> My first concern too when I saw the patch was the semantic change (but also the loss of flexibility), which is obviously a no-go. It's a convenient capability to have though, so it would be nice to get it in somehow. What if we allowed including all the VFs associated with a PF by adding an extra attribute?  e.g.:
> 
> <interface dev='eth10' type='sriov'/>
> 
> (or whatever is more appropriate in place of "sriov"). Or possibly a different element type could be used:
> 
> <pf dev='eth10'/>
> 
> (didn't want to spend time thinking of a better name than "pf"...).
> 
> At the time the network is created, this would cause libvirt to get the list of all VFs for the given PF and put them into the pool. This could be used instead of, or in combination with, the existing <interface dev='eth1'/> form. Thus the existing semantics would be preserved, the flexibility of specifying individual devices would be retained, and the desired convenience of adding all VFs of a PF with a single line would be added.
> 

I completely agree with this method, I can work on this cause next week. May I ask which method you would suggest I go forward with,

1) <interface dev='eth10' type='sriov'/>
2) Or possibly a different element type could be used: <pf dev='eth10'/>

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Laine and Daniel,

Many thanks for reviewing my patch series.

I do understand the point of loss of flexibility when we want to interleave Vfs from several PFs across virtual networks.

Regarding the changes to the network semantics, I am a little bit confused,

1) We do not require a change to the XML schema in order to support the patch series
2) Is the change required to support the modifications to the structures 'virNetworkForwardVfDef' and 'virNetworkForwardIfDef'? OR
3) Would the change be required because of the compatibility issue with interface pools?

If the concern is the compatibility issue, the patch series I have submitted takes this into consideration and will still work as before if interface pools are mentioned instead of just the PF. 
 
I completely agree with Laine's suggestion in the previous thread, I can work on this cause next week. May I ask for suggestions as to which method I should go forward with,

1) <interface dev='eth10' type='sriov'/>
2) Or possibly a different element type could be used: <pf dev='eth10'/>

Many Thanks,

Regards,
Shradha Shah



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