Sporadic loss of networking (kvm-70, e1000, tap)

[Charles Duffy <Charles_Duffy@messageone.com>]

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Several times over the last few days, I've lost network connectivity 
from one of my guests. This has happened only during interactive 
sessions in which I take an action resulting in a large screen update. I 
have tried flood pinging (only with the default, small packet size), and 
not been able to reproduce under those circumstances.

This guest is running a current RHEL5/CentOS kernel (2.6.18-53.1.13.el5) 
with clocksource=acpi_pm on the command line. The host side of the 
network is a tap device, which is joined to a bridge. Other VMs on the 
bridge still have working bidirectional networking, and dmesg on the 
host shows the relevant port on the bridge in forwarding state.

Rebooting the guest without shutting down the kvm instance does not 
resolve the issue. Powering down the VM and starting a new kvm instance 
*does* resolve the issue.

Within the guest, tcpdump sees both incoming and outgoing traffic; 
however, on the host, only traffic going *to* the guest is visible; 
traffic the guest attempts to send is not visible.

When attempting to send, none of the counters (RX packets/TX 
packets/etc) increase on the emulated e1000 device within the guest; RX 
bytes and TX bytes are both 0, and all the error counters are likewise 
zeroed. The e1000 module can be reloaded without any visible errors.

Where should I start in attempting to debug this?

[-- Attachment #2: ethtool_-d_eth0.failed.txt --]
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MAC Registers
-------------
0x00000: CTRL (Device control register)  0x00140240
      Endian mode (buffers):             little
      Link reset:                        normal
      Set link up:                       1
      Invert Loss-Of-Signal:             no
      Receive flow control:              disabled
      Transmit flow control:             disabled
      VLAN mode:                         disabled
      Auto speed detect:                 disabled
      Speed select:                      1000Mb/s
      Force speed:                       no
      Force duplex:                      no
0x00008: STATUS (Device status register) 0x80080783
      Duplex:                            full
      Link up:                           link config
      TBI mode:                          disabled
      Link speed:                        1000Mb/s
      Bus type:                          PCI
      Bus speed:                         33MHz
      Bus width:                         32-bit
0x00100: RCTL (Receive control register) 0x00008002
      Receiver:                          enabled
      Store bad packets:                 disabled
      Unicast promiscuous:               disabled
      Multicast promiscuous:             disabled
      Long packet:                       disabled
      Descriptor minimum threshold size: 1/2
      Broadcast accept mode:             accept
      VLAN filter:                       disabled
      Cononical form indicator:          disabled
      Discard pause frames:              filtered
      Pass MAC control frames:           don't pass
      Receive buffer size:               2048
0x02808: RDLEN (Receive desc length)     0x00000000
0x02810: RDH   (Receive desc head)       0x00000000
0x02818: RDT   (Receive desc tail)       0x000000FE
0x02820: RDTR  (Receive delay timer)     0x00000000
0x00400: TCTL (Transmit ctrl register)   0x0103F0FA
      Transmitter:                       enabled
      Pad short packets:                 enabled
      Software XOFF Transmission:        disabled
      Re-transmit on late collision:     enabled
0x03808: TDLEN (Transmit desc length)    0x00000000
0x03810: TDH   (Transmit desc head)      0x0000000F
0x03818: TDT   (Transmit desc tail)      0x0000000F
0x03820: TIDV  (Transmit delay timer)    0x00000000
PHY type:                                M88