Still possible to directly boot without loader?

[John Baldwin]

On Thursday 26 October 2006 10:18, Ruslan Ermilov wrote:
> On Thu, Oct 26, 2006 at 10:52:30PM +1000, Bruce Evans wrote:
> > On Thu, 26 Oct 2006, Ruslan Ermilov wrote:
> > >3)  It's currently broken even on i386; backing out
> > >   rev. 1.71 of boot2.c by jhb@ fixes this for me.
> > >
> > >: revision 1.71
> > >: date: 2004/09/18 02:07:00;  author: jhb;  state: Exp;  lines: +3 -3
> > >: A long, long time ago in a CVS branch far away (specifically, HEAD prior
> > >: to 4.0 and RELENG_3), the BTX mini-kernel used paging rather than flat
> > >: mode and clients were limited to a virtual address space of 16 megabytes.
> > >: Because of this limitation, boot2 silently masked all physical addresses
> > >: in any binaries it loaded so that they were always loaded into the first
> > >: 16 Meg.  Since BTX no longer has this limitation (and hasn't for a long
> > >: time), remove the masking from boot2.  This allows boot2 to load kernels
> > >: larger than about 12 to 14 meg (12 for non-PAE, 14 for PAE).
> > >:
> > >: Submitted by:   Sergey Lyubka devnull at uptsoft dot com
> > >: MFC after:      1 month
> > 
> > The kernel is linked at 0xc0000000 but loade din low memory, so the high
> > bits must be masked off like they used to be for the kernel to boot at all.
> > This has nothing to do with paging AFAIK.  Rev.1.71 makes no sense, since
> > BTX isn't large, and large kernels are more unbootable than before with
> > 1.71.
> > 
> The real purpose of this commit was to allow to directly "load kernels
> larger than about 12 to 14 meg (12 for non-PAE, 14 for PAE)".  (Old
> version masked high 8 bits, leaving only 2^24=16MB for the kernel.)
> 
> I have compiled GENERIC and PAE kernels; objdump(1) reports that GENERIC
> kernel has virtual "start address 0xc0449cb0", and PAE has virtual "start
> address 0xc02458f0".

Yes, KERNLOAD for PAE is 2MB and for non-PAE is 4MB (to skip PSE page 0).

> What happens here is that BTX now uses flat memory model, and by not
> masking higher bits at all, BTX attempts to load kernels at above 3G,
> which silently fails, and then jumps to the entry point located in
> "no memory" unless the machine has enough memory.
> 
> If the machine has enough physical memory, e.g. 4G, then it works (I
> think that was the case on the machine John tested this change), but
> on my test machine I only have 3G of memory, so it fails.

Actually, it should never work, as the kernel assumes it is loaded at
KERNLOAD.

> My interim solution to the problem that would still allow booting
> larger than 16MB kernels is to mask some of the higher bits.
> Currently, I mask 28 bits that gives possible 256MB which is probably
> practical.

boot2 should do whatever loader does.

-- 
John Baldwin