Re: [Qemu-devel] [PATCH v3 2/2] net: introduce command to query rx-filter information

[Markus Armbruster <armbru@redhat.com>]

Amos Kong <akong@redhat.com> writes:

> On Tue, Jul 02, 2013 at 03:27:12PM +0200, Markus Armbruster wrote:
>> Amos Kong <akong@redhat.com> writes:
>> 
>> > On Tue, Jul 02, 2013 at 11:05:56AM +0200, Markus Armbruster wrote:
>> >> Amos Kong <akong@redhat.com> writes:
>> [...]
>> >> >> This interface is abstract in the sense that it applies to all NICs.  At
>> >> >> this time, it's implemented only virtio-net implements it.  I'm
>> >> >> habitually wary of abstractions based on just one concrete instance,
>> >> >> which makes me ask:
>> >> >> 
>> >> >> 1. Ignorant question first: could the feature make sense for other NICs,
>> >> >> too, or is it specific to virtio-net?
>> >> >
>> >> > We will not. 
>> >> >
>> >> > It's ugly to check if nic is virtio-net nic in net/net.c, so I
>> >> > register the query function to NetClientInfo. Traversal the net
>> >> > client list in net/net.c, and execute query of each virtio-net
>> >> > instance in virtio-net.c
>> >> 
>> >> Implementing the feature as an optional callback is fine.
>> >> 
>> >> Let me rephrase my question: could this feature be implemented for other
>> >> NICs?  I'm *not* asking you to do that, just whether it would be
>> >> possible.
>> >> 
>> >> I'm asking because my review of the QAPI schema depends on the answer.
>> >> 
>> >> >> 2. If the former, are you reasonably sure this object will do for other
>> >> >> NICs?
>> >> >
>> >> > No.
>> >> 
>> >> I'm not sure I understand you.  Do you mean to say that the feature
>> >> could be implemented for other NICs, but RxFilterInfo would probably not
>> >> fit for them?
>> >
>> > We will not implement the feature to other NICs, no request.
>> >
>> > We notify the management of virtio-net rx-filter change, because
>> > we want to sync the the rx-filter change to macvtap device.
>> 
>> I understand there are no plans to implement this feature for other
>> NICs.  But I'm not asking whether we *want* to implement it for other
>> NICs, I'm asking whether we *could*.
>  
> In theory, we can.
>
>> Or rephrased yet another way: what exactly makes this feature applicable
>> to virtio-net only?
>
> Macvtap can only be used by virtio-net, not other emulated nic.
> It's meaningless for management to know the rx-filter change of
> non-virtio-net NICs.

I'm having trouble squaring "in theory, we can" with "meaningless".  So
I'm rephrasing my question yet again.

Do NICs other than virtio-net have rx-filters?

If yes, what have these NIC rx-filters in common, and how do they
differ?

Why would anybody want to query rx-filters?  Use cases, please.

Why is querying rx-filters "meaningless" for anything but virtio-net?
The dictionary explains "meaningless" as "having no meaning; of no
value".  Thus, for the query to be meaningless, the answer must carry no
information, or at least none of value.  Is querying rx-filters really
meaningless?  Or is it just something we don't need right now, and can't
see being needed in the future?

>> If the answer is "nothing", then we *could* implement it for other NICs.
>> Else, implementing it for other NICs would be impossible.
>> 
>> Once again, I'm not asking because I want it implemented for other
>> NICs.  I'm asking because the answer affects my review of the schema.