[Xen-users] tap:aio VBD Support.

["Dan Kendall" <Dan.Kendall@xxxxxxxxxx>]

Hi all,

I've downloaded Xen-3.0.2 and I've been trying to setup my virtual machines to 
make use of file-backed VBDs as described in this link : 
http://www.cl.cam.ac.uk/research/srg/netos/xen/readmes/user/user.html#SECTION03320000000000000000.
  I'm using the source distribution which has been built using the command :

make KERNELS=linux-2.6-xen world
make install

as detailed in the README file.

However whenever I run the xm create command with the following config I get 
the "Error: Device 2049 (vbd) could not be connected. Hotplug scripts not 
working." error that's mentioned in several other places in the list archives.

kernel = "/boot/vmlinuz-2.6.16-xen"
ramdisk = "/boot/initrd-2.6.16-xen.img"
memory = 128
name = "wwwmaster"
vif = [ '' ]
disk = [ 
'tap:aio:/data/xen_masters/www_master.img,sda1,w','tap:aio:/data/xen_swaps/www_swap1,sda2,w'
 ]
dhcp="dhcp"
root = "/dev/sda1 ro"

After some playing around I noticed the same error is reported when changing 
tap:aio to anything, eg 'fred:bert'.  I guess this device type needs some 
support in the Dom0 kernel?  The README file in the distribution describes the 
method for rebuilding the xen kernel using a different config, and sure enough 
under the XEN menu of the kernel config tool there was an unchecked option for 
the blktap tap device.  When I check this to '*' for in-built support I get the 
following compile errors:

CC      drivers/xen/blktap/xenbus.o
drivers/xen/blktap/xenbus.c: In function `frontend_changed':
drivers/xen/blktap/xenbus.c:71: warning: passing arg 1 of `xenbus_exists' makes 
integer from pointer without a cast
drivers/xen/blktap/xenbus.c:71: error: too few arguments to function 
`xenbus_exists'
drivers/xen/blktap/xenbus.c:72: warning: passing arg 1 of `xenbus_rm' makes 
integer from pointer without a cast
drivers/xen/blktap/xenbus.c:72: error: too few arguments to function `xenbus_rm'
drivers/xen/blktap/xenbus.c:80: warning: passing arg 1 of `xenbus_gather' makes 
integer from pointer without a cast
drivers/xen/blktap/xenbus.c:96: warning: implicit declaration of function 
`xenbus_dev_ok'
drivers/xen/blktap/xenbus.c:101: error: too few arguments to function 
`xenbus_transaction_end'
drivers/xen/blktap/xenbus.c: In function `blkback_probe':
drivers/xen/blktap/xenbus.c:154: warning: passing arg 1 of `xenbus_gather' 
makes integer from pointer without a cast
drivers/xen/blktap/xenbus.c:163: warning: passing arg 1 of `xenbus_exists' 
makes integer from pointer without a cast
drivers/xen/blktap/xenbus.c:163: error: too few arguments to function 
`xenbus_exists'
make[5]: *** [drivers/xen/blktap/xenbus.o] Error 1
make[4]: *** [drivers/xen/blktap] Error 2
make[3]: *** [drivers/xen] Error 2
make[2]: *** [drivers] Error 2
make[2]: Leaving directory `/home/xxxxx/xen-3.0.2/linux-2.6.16-xen'
make[1]: *** [build] Error 2
make[1]: Leaving directory `/home/xxxxx/xen-3.0.2'
make: *** [linux-2.6-xen-install] Error 2

Am I right in thinking I need this support in the kernel to use the tap:aio 
file-backed method?  If so, does Xen-3.0.2 support this properly?  And if not 
what version of Xen do I need to use to support the method described in the 
user manual?  FWIW I have successfully used the 'file:' method for file-backed 
VBDs but I envisage using a largish number of VMs and using the loop device may 
prove problematic.

Many thanks for any pointers.

Dan





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