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Re: [Qemu-discuss] Macvtap statistics

From: Vlad Yasevich
Subject: Re: [Qemu-discuss] Macvtap statistics
Date: Mon, 21 Mar 2016 08:56:37 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

On 03/21/2016 05:19 AM, Venkateswara Rao Dokku wrote:
> Hi,
> I am a little confused about what is RX & TX on the TAP & guest inteface.
> When we launch a VM, with 1 PNIC attached to the guest as a Macvtap bridge,
> the traffic can flow through 2 directions.
> 1. From the Guest VM's interface to the PNIC via the TAP interface
> 2. From the outside network to the guest interface via the TAP interface.
> So, which is considered as Rx & which is considered as Tx?
> Is the meaning is same in both the interfaces guest & TAp on host. i.e. Rx
> counter on TAP & Rx counter guest interface are same or Rx on TAP becomes
> Tx on guest interface & vice-versa.
> or, Is the TAP interface on host is simply the replica of the guest
> interface?

Don't confuse the issues by using references to TAP.  Macvtap has glued
TAP into it.

Macvtap stats are different from TAP stats.

Macvtap counts packets received from PNIC or another bridged macvtap in RX 
This should mimic stats on the guest device, but may be a bit different if 
has to drop packets.
Packets sent by the guest will be counted in Macvtap TX stats.

Now, to make matters more confusing, when you are using TAP + bridge, the stats 
the TAP device are reversed.   Packets sent by the guest are counted as RX on 
TAP and
TX on guest, and packets received from external source are counted as TX on TAP 
and RX
on guest.


> Thanks in advance.
> .

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