Code Sample for Reading Values from Hyper-V KVP component on Linux (aka KVP Data Exchange) - hyper-v

Question

Hyper-V includes a KVP component that transmits key / value pairs between the host and a guest VM.
Code samples for sending and receiving values are available for Windows Guests in PowerShell in WMI.
However, my guest is using a Linux version of this service.
Where can I find a sample Linux script that queries this service for key / value pairs?

Answer 1

Key details from a blog entry I wrote covering the problem. (I could not find the answer elsewhere):
First, make sure you have the KVP service installed.
KVP data is transferred to the Linux file system through the collaboration of a kernel driver and a user mode daemon.
The [KVP driver code], hv_kvp.c, is compiled into the hv_util kernel module Source). Since the driver is part of the Linux kernel code, it is provided by default with recent versions of common Linux distributions. E.g.
[root#centos6-4-hv ~]# cat /etc/*-release
CentOS release 6.4 (Final)
CentOS release 6.4 (Final)
CentOS release 6.4 (Final)
[root#centos6-4-hv ~]# modinfo -F filename hv_utils
/lib/modules/2.6.32-358.el6.i686/kernel/drivers/hv/hv_utils.ko
However, it is the usermode daemon, hv_kvp_daemon, that copies KVP data to the system. On startup, hv_kvp_daemon creates files to store kvp data under
/var/lib/hyperv (source). Each file is known as a 'pool', and there is a file for each data pool. E.g.
[root#centos6-4-hv hyperv]# ls -al /var/lib/hyperv/
total 36
drwxr-xr-x. 2 root root 4096 Sep 11 21:33 .
drwxr-xr-x. 16 root root 4096 Sep 10 13:59 ..
-rw-r--r--. 1 root root 2560 Sep 10 17:05 .kvp_pool_0
-rw-r--r--. 1 root root 0 Sep 10 14:02 .kvp_pool_1
-rw-r--r--. 1 root root 0 Sep 10 14:02 .kvp_pool_2
-rw-r--r--. 1 root root 28160 Sep 10 14:02 .kvp_pool_3
-rw-r--r--. 1 root root 0 Sep 10 14:02 .kvp_pool_4
The prefix of each file is the pool number. This corresponds to the KVP source. E.g. remember that source '0' is used for transmitting data from host to guest? That means our KVP data is in /var/lib/hyperv/.kvp_pool_0. E.g.
[root#centos6-4-hv hyperv]# cat /var/lib/hyperv/.kvp_pool_0
cloudstack-vm-userdatausername=root;password=1pass#word1[root#centos6-4-hv hyperv]#
These KVP data files contain an array of key / value pairs. Each is a byte array of a fixed size. (source)
/*
* Maximum key size - the registry limit for the length of an entry name
* is 256 characters, including the null terminator
*/
#define HV_KVP_EXCHANGE_MAX_KEY_SIZE (512)
/*
* bytes, including any null terminators
*/
#define HV_KVP_EXCHANGE_MAX_VALUE_SIZE (2048)
The byte array contains a UTF-8 encoded string, which is padded out to the max size with null characters. However, null string termination is not guaranteed (see kvp_send_key).
Provided there is only one key and the key name known, the easiest way to parse the file is to use sed. To remove null characters and the key name used in our example, you would use the following:
[root#centos6-4-hv hyperv]# cat /var/lib/hyperv/.kvp_pool_0 | sed 's/\x0//g' | sed 's/cloudstack-vm-userdata//g' > userdata
[root#centos6-4-hv hyperv]# more userdata
username=root;password=1pass#word1

Answer 2

Here is a bash script to read the key-value pairs for a given file:
#!/bin/bash
fname=$1
echo "Reading $fname"
nb=$(wc -c < $1)
nkv=$(( nb / (512+2048) ))
for n in $(seq 0 $(( $nkv - 1 )) ); do
offset=$(( $n * (512 + 2048) ))
k=$(dd if=$fname count=512 bs=1 skip=$offset status=none | sed 's/\x0.*//g')
v=$(dd if=$fname count=2048 bs=1 skip=$(( $offset + 512 )) status=none | sed 's/\x0.*//g')
echo "$k = $v"
done

Answer 3

Same functionality in java:
String guest_param_file="/var/lib/hyperv/.kvp_pool_3";
if (new File(guest_param_file).exists())
{
try
{
BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(guest_param_file),"UTF-8"));
char [] ckey=new char[512] ;
char [] cvalue=new char[2048] ;
while (true)
{
int charcount=br.read(ckey);
if (charcount==-1)
{
break;
}
br.read(cvalue);
String key=new String(ckey).trim();
String value=new String(cvalue).trim();
System.out.println( key+" = "+value);
}
br.close();
}
catch (UnsupportedEncodingException ex)
{
}
catch (FileNotFoundException ex)
{
}
catch (IOException ex)
{
}
}