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

To: Andrew Morton <akpm@xxxxxxxx>
Subject: [Xen-devel] Re: [patch 7/8] Add a bootparameter to reserve high linear address space.
From: Zachary Amsden <zach@xxxxxxxxxx>
Date: Thu, 03 Aug 2006 00:33:10 -0700
Cc: jeremy@xxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, chrisw@xxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx
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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

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.


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