igb is broken, even across reboots, at r312294

Today I updated my machine from 311787 to 312294.  After the update,
my igb ports can pass no traffic.  If I reboot into kernel.old, they
still can't pass any traffic.  They won't even work in the PXE ROM.  I
have to power off, pull the power cables, then boot into kernel.old
before they'll work.  This behavior is repeatable.

$ pciconf -lv
...
igb0_at_pci0:1:0:0:        class=0x020000 card=0x34dc8086 chip=0x10a78086 rev=0x02
hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82575EB Gigabit Network Connection'
    class      = network
    subclass   = ethernet
igb1_at_pci0:1:0:1:        class=0x020000 card=0x34dc8086 chip=0x10a78086
rev=0x02 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82575EB Gigabit Network Connection'
    class      = network
    subclass   = ethernet
...

$ dmesg # on 311787, it's identical whether or not the igb ports are working
...
igb0: <Intel(R) PRO/1000 Network Connection, Version - 2.5.3-k> port
0x2020-0x203f mem 0xb1b20000-0xb1b3ffff,0xb1b44000-0xb1b47fff irq 40
at device 0.0 on pci1
igb0: Using MSIX interrupts with 5 vectors
igb0: Ethernet address: 00:1e:67:25:71:bc
igb0: Bound queue 0 to cpu 0
igb0: Bound queue 1 to cpu 1
igb0: Bound queue 2 to cpu 2
igb0: Bound queue 3 to cpu 3
igb0: netmap queues/slots: TX 4/1024, RX 4/1024
igb1: <Intel(R) PRO/1000 Network Connection, Version - 2.5.3-k> port
0x2000-0x201f mem 0xb1b00000-0xb1b1ffff,0xb1b40000-0xb1b43fff irq 28
at device 0.1 on pci1
igb1: Using MSIX interrupts with 5 vectors
igb1: Ethernet address: 00:1e:67:25:71:bd
igb1: Bound queue 0 to cpu 4
igb1: Bound queue 1 to cpu 5
igb1: Bound queue 2 to cpu 6
igb1: Bound queue 3 to cpu 7
igb1: netmap queues/slots: TX 4/1024, RX 4/1024
...

$ dmesg # on 312294, when the igb ports are not working
...
igb0: <Intel(R) PRO/1000 PCI-Express Network Driver> port
0x2020-0x203f mem 0xb1b20000-0xb1b3ffff,0xb1b44000-0xb1b47fff irq 40
at device 0.0 on pci1
igb0: attach_pre capping queues at 4
igb0: using 1024 tx descriptors and 1024 rx descriptors
igb0: msix_init qsets capped at 4
igb0: pxm cpus: 8 queue msgs: 9 admincnt: 1
igb0: using 4 rx queues 4 tx queues
igb0: Using MSIX interrupts with 5 vectors
igb0: allocated for 4 tx_queues
igb0: allocated for 4 rx_queues
igb0: Ethernet address: 00:1e:67:25:71:bc
igb0: netmap queues/slots: TX 4/1024, RX 4/1024
igb1: <Intel(R) PRO/1000 PCI-Express Network Driver> port
0x2000-0x201f mem 0xb1b00000-0xb1b1ffff,0xb1b40000-0xb1b43fff irq 28
at device 0.1 on pci1
igb1: attach_pre capping queues at 4
igb1: using 1024 tx descriptors and 1024 rx descriptors
igb1: msix_init qsets capped at 4
igb1: pxm cpus: 8 queue msgs: 9 admincnt: 1
igb1: using 4 rx queues 4 tx queues
igb1: Using MSIX interrupts with 5 vectors
igb1: allocated for 4 tx_queues
igb1: allocated for 4 rx_queues
igb1: Ethernet address: 00:1e:67:25:71:bd
igb1: netmap queues/slots: TX 4/1024, RX 4/1024
...

Any ideas?

-Alan