Subject: Re: [PATCH net] veth: Add updating of trans_start

From: Jay Vosburgh <[email-masked]>
Date: Fri, 17 Jun 2022 09:42:55 -0700

Jakub Kicinski <[email-masked]> wrote:

>On Thu, 16 Jun 2022 12:26:30 -0700 Jay Vosburgh wrote:
>> 	Since commit 21a75f0915dd ("bonding: Fix ARP monitor validation"),
>> the bonding ARP / ND link monitors depend on the trans_start time to
>> determine link availability.  NETIF_F_LLTX drivers must update trans_start
>> directly, which veth does not do.  This prevents use of the ARP or ND link
>> monitors with veth interfaces in a bond.
>
>Why is a SW device required to update its trans_start? trans_start is
>for the Tx hang watchdog, AFAIK, not a general use attribute. There's
>plenty of NETIF_F_LLTX devices, are they all broken? 

	In this case, it's to permit the bonding ARP / ND monitor to
function if that software device (veth in this case) is added to a bond
using the ARP / ND monitor (which relies on trans_start, and has done so
since at least 2.6.0).  I'll agree it's a niche case; this was broken
for veth for quite some time, but veth + netns is handy for software
only test cases, so it seems worth doing.

	I didn't exhaustively check all LLTX drivers, but, e.g., tun
does update trans_start:

drivers/net/tun.c:

       /* NETIF_F_LLTX requires to do our own update of trans_start */
        queue = netdev_get_tx_queue(dev, txq);
        txq_trans_cond_update(queue);

	-J

---
	-Jay Vosburgh, [email-masked]