Date: Thu, 26 Oct 2006 18:42:40 +0400 From: Ruslan Ermilov <ru@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-stable@freebsd.org, Stefan Bethke <stb@lassitu.de>, Bruce Evans <bde@zeta.org.au>, Bruce Evans <bde@freebsd.org> Subject: Re: Still possible to directly boot without loader? Message-ID: <20061026144240.GB1219@rambler-co.ru> In-Reply-To: <200610261028.10680.jhb@freebsd.org> References: <3A8131D4-881E-4873-A682-543A1A88C063@lassitu.de> <20061026222234.P2553@epsplex.bde.org> <20061026141830.GA790@rambler-co.ru> <200610261028.10680.jhb@freebsd.org>
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On Thu, Oct 26, 2006 at 10:28:09AM -0400, John Baldwin wrote:
> 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.
>
But this would be a regression, since loader(8) does the following,
in the ELF32 case:
: 0 edoofus:ttyp2:/sys/boot/i386/libi386 >grep -w entry elf32_freebsd.c
: vm_offset_t entry, bootinfop, modulep, kernend;
: entry = ehdr->e_entry & 0xffffff;
: printf("Start @ 0x%lx ...\n", entry);
: __exec((void *)entry, boothowto, bootdev, 0, 0, 0, bootinfop, modulep, kernend);
Cheers,
--
Ruslan Ermilov
ru@FreeBSD.org
FreeBSD committer
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