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

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Message-Id: <20060803004144.554d9882.akpm@osdl.org>
Date:	Thu, 3 Aug 2006 00:41:44 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Zachary Amsden <zach@...are.com>
Cc:	jeremy@...source.com, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	jeremy@...p.org, chrisw@...s-sol.org
Subject: Re: [patch 7/8] Add a bootparameter to reserve high linear address
 space.

On Thu, 03 Aug 2006 00:33:10 -0700
Zachary Amsden <zach@...are.com> 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.

The comment says "must".  If that's true then printing a what-you-did-wrong
message and halting is appropriate.

But whatever.  The issue is flagged and I'm happy to leave it in Jeremy's
lap.  
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