linux-kernel - Re: [PPC64/Power7 - 2.6.35-rc5] Bad relocation warnings whileBuilding a CONFIG_RELOCATABLE kernel with CONFIG

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Message-ID: <reloc-2010-07-19-2@mdm.bga.com>
Date:	Mon, 19 Jul 2010 06:35:16 -0500
From:	Milton Miller <miltonm@....com>
To:	Alexander Graf <agraf@...e.de>
Cc:	Subrata Modak <subrata@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PPC64/Power7 - 2.6.35-rc5] Bad relocation warnings whileBuilding a CONFIG_RELOCATABLE kernel with CONFIG_ISERIES enabled

I wrote:
> On Mon Jul 19 2010 at about 03:36:51 EST, Alexander Graf wrote:
> > On 19.07.2010, at 03:11, Benjamin Herrenschmidt wrote:
> > 
> > > On Thu, 2010-07-15 at 17:05 +0530, Subrata Modak wrote:
> > > > commit e62cee42e66dcca83aae02748535f62e0f564a0c solved the problem for
> > > > 2.6.34-rc6. However some other bad relocation warnings generated against
> > > > 2.6.35-rc5 on Power7/ppc64 below:
> > > >
> > > > MODPOST 2004 modules^M
> > > > WARNING: 2 bad relocations^M
> > > > c000000000008590 R_PPC64_ADDR32 .text+0x4000000000008460^M
> > > > c000000000008594 R_PPC64_ADDR32 .text+0x4000000000008598^M
> > >
> > > I think this is KVM + CONFIG_RELOCATABLE. Caused by:
> > >
> > > .global kvmppc_trampoline_lowmem
> > > kvmppc_trampoline_lowmem:
> > > .long kvmppc_handler_lowmem_trampoline - CONFIG_KERNEL_START
> > >
> > > .global kvmppc_trampoline_enter
> > > kvmppc_trampoline_enter:
> > > .long kvmppc_handler_trampoline_enter - CONFIG_KERNEL_START
> > >
> > > Alex, can you turn these into 64-bit on ppc64 so the relocator
> > > can grok them ?
> > 
> > If I turn them into 64-bit, will the values be > RMA? In that case
> > things would break anyways. How does relocation work on PPC? Are the
> > first few megs copied over to low memory? Would I have to mask anything
> > in the above code to make sure I use the real values? 
> >
> > Alex
> >
> 
> You can still do the subtraction, but you have to allocate 64 bits for
> storage.  Relocatable ppc64 kernels work by adjusting PPC64_RELOC_RELATIVE
> entries during early boot (reloc in reloc_64.S called from head_64.S).
> 
> The code purposely only supports 64 bit relative addressing.

Oh yea, and for book-3s, the code copies from 0x100 to __end_interrupts
in arch/powerpc/kernel/exceptions-64s.h down to the real 0, but the rest
of the kernel is at some disjointed address.  The interrupt will go to
the copy at the real zero.  Any references to code outside that region
must be done via a full indrect branch (not a relative one), simiar to
the secondary startup (via following the function pointer in a descriptor
set in very low memory), or syscall entry and exception vectors via paca.

Book-3e (64 and 32 bit) are different.  I forget how classic 32 bit works,
it may still have a < 32MB limitation.

milton
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