Re: [Xen-users] [Fwd: DHCP and DomUs troubles]

[Arnaud JAYET <ajayet@xxxxxxx>]

Fajar A. Nugraha a écrit :
 > You could hack /etc/xen/scripts/vif-bridge, preferably be creating a
> new file and use it on domU config file (per domU) or set it to be
> default on xend-config.sxp (global). In my case, I hacked it to
> automatically restart snmp every time a new interface is activated.
Interesting. But how can you specify a specific network or vif script 
inside a domU .cfg config file ?
I don't see any explanation about this in Xen user documentation.


To automate domU tx checksum disabling i do the following hack :

in /etc/xen/scripts/xen-network-common.sh, i added the ehttool line at 
end of add_to_bridge() function :
# Usage: add_to_bridge bridge dev
add_to_bridge () {
    local bridge=$1
    local dev=$2

    # Don't add $dev to $bridge if it's already on a bridge.
    if [ -e "/sys/class/net/${bridge}/brif/${dev}" ]; then
        ip link set ${dev} up || true
        return
    fi
    brctl addif ${bridge} ${dev}
    ip link set ${dev} up

    ### disabling tx checksum for vif x.y, DHCP trouble
    ethtool -K ${dev} tx off
}


When creating the domU, it's vif X.Y interface has now tx checksum off 
and my dhcp's request problem inside domU disappears. my DomU has now 
its dynamic IP address at boot.
I also noticed that the ehttool line in /etc/network/interfaces inside 
the domU is useless. Without this line, domU's eth0 tx checksum is on 
but DHCP request works.
domU (ID 17) boot :
...
...
Listening on LPF/eth0/00:16:3e:7f:bf:11
Sending on   LPF/eth0/00:16:3e:7f:bf:11
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 4
DHCPOFFER from 192.168.115.1
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPACK from 192.168.115.1
bound to 192.168.115.107 -- renewal in 19160 seconds.
done.
...
...


domU# ethtool -k eth0
Offload parameters for eth0:
Cannot get device rx csum settings: Operation not supported
Cannot get device flags: Operation not supported
rx-checksumming: off
==> tx-checksumming: on
scatter-gather: on
tcp segmentation offload: on
udp fragmentation offload: off
generic segmentation offload: off
large receive offload: off


domO# ethtool -k vif17.0
Offload parameters for vif17.0:
Cannot get device rx csum settings: Operation not supported
Cannot get device flags: Operation not supported
rx-checksumming: off
==> tx-checksumming: off
scatter-gather: on
tcp segmentation offload: on
udp fragmentation offload: off
generic segmentation offload: off
large receive offload: off

----

A more general question above my specific DHCP problem. Is it a good 
idea to always disable tx (and rx ?) interface checksum for DomUs 
(inside domU and inside dom0 vif interfaces) ?
I red in the Xen mailing list some performance troubles due to domU tx 
checksums.
In recent Xen version (like 3.4) does the tx/tx checksum are disabled by 
default in domO vif X.Y interfaces and domUs ?

Thank you for your help Fajar.
I hope this thread could help Debian Lenny users.

Arnaud


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