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

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Message-ID: <44D1A6B6.8040003@vmware.com>
Date:	Thu, 03 Aug 2006 00:33:10 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Jeremy Fitzhardinge <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.

Andrew Morton wrote:
> On Wed, 02 Aug 2006 17:25:17 -0700
> Jeremy Fitzhardinge <jeremy@...source.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.

Zach

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