[Xen-devel] Re: [patch 7/8] Add a bootparameter to reserve high linear address space.

[Zachary Amsden <zach@xxxxxxxxxx>]

Andrew Morton wrote:
> On Wed, 02 Aug 2006 17:25:17 -0700
> Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx> wrote:
>
>   
> > +               /*
> > +                * reservetop=size reserves a hole at the top of the kernel
> > +                * address space which a hypervisor can load into later.
> > +                * Needed for dynamically loaded hypervisors, so relocating
> > +                * the fixmap can be done before paging initialization.
> > +                * This hole must be a multiple of 4M.
> > +                */
> > +               else if (!memcmp(from, "reservetop=", 11)) {
> > +                       unsigned long reserve = memparse(from+11, &from);
> > +                       reserve &= ~0x3fffff;
> > +                       reserve_top_address(reserve);
> > +               }
> >     
> I assume that this argument will normally be passed in via the hypervisor
> rather than by human-entered information?
>
> In which case, perhaps a panic would be a more appropriate response to a
> non-multiple-of-4M.
>
> Either way, rounding the number down rather than up seems wrong...
>   
Agree on the rounding issue - but is a panic really correct?  Perhaps we 
should not round at all.
The presumption is actually that this is human or script entered 
information.  A runtime loaded hypervisor module has no way to tweak or 
toggle the boot parameters, as it hasn't yet been loaded.  It could be 
that a human operator wants to make room for it.  Giving the operator a 
panic is not the most friendly thing to do - logging the failure on 
module load is much nicer.  And such a runtime loaded hypervisor must be 
fully virtualizing anyway, so even if the argument is wrong and doesn't 
give the hypervisor enough space to load, no damage is done - the 
operator just resets the parameter and reboots.
Zach


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