em0 link fail

[Michael Butler]

On June 1st, I was able to do my monthly laptop ZFS snap-shot/back-up
(using "zfs snapshot -r zroot at backup; zfs send -R >nfs-filesys"). Now I
can't without the em0 interface stalling :-(

On a guess, I tried reverting SVN r335303 but that didn't help.

em0: <Intel(R) PRO/1000 Network Connection> port 0xf080-0xf09f mem
0xf7e00000-0xf7e1ffff,0xf7e39000-0xf7e39fff irq 20 at device 25.0 on pci0
em0: attach_pre capping queues at 1
em0: using 1024 tx descriptors and 1024 rx descriptors
em0: msix_init qsets capped at 1
em0: PCIY_MSIX capability not found; or rid 0 == 0.
em0: Using an MSI interrupt
em0: allocated for 1 tx_queues
em0: allocated for 1 rx_queues
em0: Ethernet address: f0:1f:af:66:95:7e
em0: netmap queues/slots: TX 1/1024, RX 1/1024
em0: link state changed to UP

 [ initiate "zfs send" ]

em0: TX(0) desc avail = 41, pidx = 172
em0: link state changed to DOWN
em0: TX(0) desc avail = 1024, pidx = 0
em0: TX(0) desc avail = 1024, pidx = 0

 .. ad nauseum ..

"ifconfig em0 down; ifconfig em0 up" doesn't help.

Any hints?

	imb