Still possible to directly boot without loader?

Bruce Evans bde at
Wed Nov 1 13:45:00 UTC 2006

On Mon, 30 Oct 2006, John Baldwin wrote:

> On Thursday 26 October 2006 15:54, Ruslan Ermilov wrote:
>> On Thu, Oct 26, 2006 at 03:42:34PM -0400, John Baldwin wrote:
>>> On Thursday 26 October 2006 15:18, Ruslan Ermilov wrote:
>>>> On Thu, Oct 26, 2006 at 11:38:24AM -0400, John Baldwin wrote:
>>> Sorry, I meant that both boot2 and loader should follow your proposal of
> masking 28 bits.
>>> Just masking the top 4 bits is probably sufficient.
>> :-)
>> OK, I'll craft a patch tomorrow.  This will also require patching at least
>> sys/boot/common/load_elf.c:__elfN(loadimage), maybe something else.
>> I think we could actually mask 30 bits; that would allow to load 1G kernels,
>> provided that sufficient memory exists.
> Actually, please mask 4 bits.  Not all kernels run at 0xc0000000.  You can
> adjust that address via 'options KVA_PAGES'.  I know of folks who run kernels
> at 0xa0000000 for example because they need more KVA.  This is part of why I

They can probably use 0x80000000, but it's not obvious how to get exactly
that from KVA_PAGES.

> really don't like the masking part, though I'm not sure there's a way to
> figure out KERNBASE well enough to do the more correct 'pa = addr - KERNBASE'
> rather than 'pa = addr & 0x0fffffff'.

The masking hack is probably only needed for aout.  For elf,
objdump -h /kernel says:

% Sections:
% Idx Name          Size      VMA       LMA       File off  Algn
% ...
%   4 .text         002853e0  c043b510  c043b510  0003b510  2**4

so KERNBASE = LMA - <File off> for at least this kernel.  boot2 now
loads the text section from file offset <File off> to address LMA(masked).
I think it just needs to load at an address that is the same mod
PAGE_SIZE as LMA or VMA (these must agree mod PAGE_SIZE), provided it
adjusts the entry address to match.


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