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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 2/2] net: introduce command to query rx-filter information
Date: Fri, 24 May 2013 12:05:12 -0600
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On 05/24/2013 10:12 AM, Michael S. Tsirkin wrote:
>>>>> Event message contains the net client name, management might only want
>>>>> to query the single net client.
>>>> The client can do the filtering itself.

>> I'm not sure I buy the responsiveness argument.  Sure, the fastest I/O
>> is no I/O, but whether you read and parse 100 bytes or 1000 from a Unix
>> domain socket once in a great while shouldn't make a difference.

And the time spent malloc'ing the larger message to send from qemu, as
well as the time spent malloc'ing the libvirt side that parses the qemu
string into C code for use, and the time spent strcmp'ing every entry to
find the right one...

It really IS more efficient to filter as low down in the stack as
possible, once it is determined that filtering is desirable.

Whether filtering makes a difference in performance is a different
question - you may be right that always returning the entire list and
making libvirt do its own filtering will still not add any more
noticeable delay compared to libvirt doing a filtered query, if the
bottleneck lies elsewhere (such as libvirt telling macvtap its new

>> My main concern is to keep the external interface simple.  I'm rather
>> reluctant to have query commands grow options.
>> In a case where we need the "give me everything" query anyway, the "give
>> me this particular part" option is additional complexity.  Needs
>> justification, say arguments involving throughput, latency or client
>> complexity.
>> Perhaps cases exist where we never want to ask for everything.  Then the
>> "give me everything" query is useless, and the option should be
>> mandatory.

For this _particular_ interface, I'm not sure whether libvirt will ever
use an unfiltered query - that is, the rx-filter query will probably
always be invoked in response to an event, at which point libvirt only
cares about the filter status of the nic named in the event.  And
ultimately libvirt knows what nics it passed to the guest, so even if
there isn't a global query and I guessed wrong about libvirt never
wanting all state at once, it would still be possible for libvirt to
iterate over one query per nic.  On the other hand, consistency with
other query-* QMP commands says that most of them return as much
information as possible all the time, and generally libvirt likes this -
even the newly-added query-command-line-options has a filtering option,
but current libvirt.git only uses it once in global mode rather than
once-per-option in filtered mode.

> We need the query for macvtap devices.  We don't need it
> for tap devices. In that case you don't want tap device info.
> Maybe some libvirt guys can tell us whether they prefer
> a per device query or a global one with info for all NICs?

Libvirt can cope either way.  I personally like the idea of allowing
both global and filtered queries, without second-guessing what
management apps will prefer to use, and don't think filtering adds that
much complexity.  But if you want to insist on avoiding filtering, I'd
rather have a global query than a mandatory name argument, for
consistency with other query-* commands, even if libvirt then ends up
doing its own filtering.

If we get introspection into qemu 1.6 at the same time as the new query
for rx-filters, it really won't matter whether you start with
global-only or mandatory name-only; either way, if we change our mind
and add the other mode in qemu 1.7, libvirt will still be able to use
introspection to determine whether the argument is present in one
direction (going from global-only to optional filtering), or whether the
argument has been made optionl in the other direction (going from
mandatory name to optional global).

> I think for HMP it's best to have nic optional.

This is true, no matter what we decide for QMP.

> Is it a good idea to make QMP match HMP closely?

QMP has to provide enough information for HMP to do its job.  How will
HMP do global listing if QMP doesn't provide a way to get all the
devices at once?  Remember, libvirt knows what devices it told qemu to
create, but I don't know that HMP has the same visibility into the list
of possible devices that can be queried.  So you may need a global mode
to begin with.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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