How to load backend drivers in 4.2

How to load backend drivers in 4.2

[Olaf Hering]

The xencommons runlevel script has a few modprobe calls to load drivers
for dom0. Recently also the backend drivers for vbd and vif were added.
Unfortunately without an explanation why that is (suddenly) needed.

Now that I have been hitting such missing backend driver issue as well
with a xenlinux based dom0 kernel I wonder how to handle the situation.

The pvops kernel has 3 backend drivers which have module aliases:
  xen-netback ; alias:          xen-backend:vif
  xen-blkback ; alias:          xen-backend:vbd
  xen-pciback ; alias:          xen-backend:pci
These aliases have been in the tree since at least 3.2, which is a long
time.

The sles11sp2 kernel has more backend drivers, but they have no alias:
  netbk
  blkbk
  xen-scsibk
  usbbk
  tpmbk
  pciback

I'm sure adding ad a MODULE_ALIAS() entry to netbk, blkbk and pciback to
match mainline is trivial.

I wonder why libxl does not do a 'modprobe xen-backend:vif &>/dev/null' if
it is about to configure an interface for the first time? The current
error message when a backend driver is missing is just not helpful.

So what should be done in xencommons for 4.2? Add a dumb loop like this?

for m in xen-backend:{vif,vbd,pci} netbk blkbk pciback xen-scsibk usbbk tpmbk
do
        modprobe $m &> /dev/null
done

Or should rather libxl take care of loading these things?


Olaf

Re: How to load backend drivers in 4.2

[Jan Beulich]

>>> On 10.08.12 at 16:10, Olaf Hering <olaf@aepfle.de> wrote:
> The xencommons runlevel script has a few modprobe calls to load drivers
> for dom0. Recently also the backend drivers for vbd and vif were added.
> Unfortunately without an explanation why that is (suddenly) needed.
> 
> Now that I have been hitting such missing backend driver issue as well
> with a xenlinux based dom0 kernel I wonder how to handle the situation.

See also http://lists.xen.org/archives/html/xen-devel/2012-08/msg00717.html.

> The pvops kernel has 3 backend drivers which have module aliases:
>   xen-netback ; alias:          xen-backend:vif
>   xen-blkback ; alias:          xen-backend:vbd
>   xen-pciback ; alias:          xen-backend:pci
> These aliases have been in the tree since at least 3.2, which is a long
> time.
> 
> The sles11sp2 kernel has more backend drivers, but they have no alias:
>   netbk
>   blkbk
>   xen-scsibk
>   usbbk
>   tpmbk
>   pciback
> 
> I'm sure adding ad a MODULE_ALIAS() entry to netbk, blkbk and pciback to
> match mainline is trivial.

They're just not there because they only got introduced in 3.1.
Adding them of course is a nobrainer.

> I wonder why libxl does not do a 'modprobe xen-backend:vif &>/dev/null' if
> it is about to configure an interface for the first time? The current
> error message when a backend driver is missing is just not helpful.
> 
> So what should be done in xencommons for 4.2? Add a dumb loop like this?
> 
> for m in xen-backend:{vif,vbd,pci} netbk blkbk pciback xen-scsibk usbbk tpmbk
> do
>         modprobe $m &> /dev/null
> done
> 
> Or should rather libxl take care of loading these things?

The outcome was that after 4.2 libxl should be doing this. For
4.2, we just need to get the list completed.

Jan