Re: Baffled by multipage_allocation_permitted()

[Dave McCracken <dcm@mccr.org>]

On Tuesday 16 June 2009, Keir Fraser wrote:
> On 16/06/2009 10:19, "Jan Beulich" <JBeulich@novell.com> wrote:
> >> We can just get rid of it I think. There are arguably worse other things
> >> we already allow, like any guest can allocate low memory by specifying a
> >> small value of XENMEMF_address_bits().
> >
> > Wouldn't it be better to tighten things rather than relaxing them?
>
> It'll need policy to be expressed and pushed down from the toolstack
> somehow. Noone's really thought about it too much. In this case I think I
> will keep the checking macro actually, and simply disallow allocations
> greater than order-9 (2MB superpage) unless the domain has direct hardware
> access capability.

Thanks for the change.  That resolves my problem.

Out of curiosity, why is it necessary to restrict the size of multipage 
allocations at all?  I'd think the memory size limits would adequately protect 
against any guest going wild with it.  What harm could come from, for example, 
a guest asking for a 16M contiguous page?  I don't know of any current need 
for such a thing.  I'm just wondering why we restrict it.

Thanks,
Dave McCracken
Oracle Corp.