RE: [Xen-users] Could not enable shadow paging for domain?

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Tom Horsley
> Sent: 18 December 2006 13:50
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] Could not enable shadow paging for domain?
> 
> > The "Count not able shadow paging" is probably related to 
> the previous
> > "cannot not allocate" error. If you can't allocate the 
> memory, then you
> > can't enable shadow paging on the memory as it's not been 
> allocated...
> > :-(
> 
> I don't think that's a "previous" error, I think the cannot 
> allocate is merely
> what gets reported outside the log and the shadow paging error is the
> more detailed info that appears in the log for the same error.

Well, to be honest, I think it's a bit of both - the REAL error is
"Can't allocate memory" (or something like that: I looked at
.../xen/arch/x86/mm/shadow/common.c: shadow_enable() [which is called
indirectly from xc_hvm_build_internal() that gives the "Could not enable
shadow..." message], and the shadow_enable() has several memory
allocations that can fail, resulting in ENOMEM... So the failure to
allocate memory is the "first" error, but the consequence is that it's
failing to enable shadow paging and in turn fails to create the domain.
It may be that there's more to it than that, but without digging further
down the tree of functions, I'm not able to say - and it would require
having some more information on where it fails to help here - adding
some printk's to the shadow_enable function would be the first part,
then further down to get a precise spot (or adding BUG_ON() statement to
"crash" the system and get a stack-dump to see where it came from in
some allocation routine, but that could be dangerous as some apps try to
allocate a large chunk then smaller if it fails). 

But I would start with my original suggestion of locking down Dom0
memory and not relying on "ballooning" - it's been known to fail before.
See below for more details on how to do that. 

If it still fails, please provide:
1. "xm info" output
2. "xm list" output

> 
> > How much memory do you give Dom0? I'd say giving Dom0 256MB 
> would be a
> > good idea, rather than rely on the auto-baloon feature that 
> is indeed a
> > bit more tricky to make work. 
> 
> I haven't yet stumbled across any documentation that tells me 
> how to limit dom0
> to a subset of the memory. What do I configure where to do that?

On the Xen-line in grub.conf (something like "kernel .../xen.gz ... ")
you add "dom0_mem=size". Size is by default in kilobytes, but you can
add "M" or "G" to make it megabytes or kilobytes (not sure if "b" makes
it bytes, but I'm sure M is what you want anyways). 

So dom0_mem=256M should work well (that's what I use!). 

--
Mats
> 
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