Re: [PATCH] [ARM] Use AT() in the linker script to create correctprogram headers

[Dave Martin]

On Mon, Oct 01, 2012 at 12:35:43PM -0600, Jason Gunthorpe wrote:
> On Mon, Oct 01, 2012 at 06:56:47PM +0100, Dave Martin wrote:
> 
> > > > If the kernel is intended to be loadable at a physical address which is
> > > > not statically known, no ELF loader that does not ignore the ELF
> > > > phdr
> > > 
> > > In this case you can't really use a standard ELF loader to load the
> > > kernel so, LOAD_OFFSET = 0 is fine. My case is using an ELF loader,
> > > and I have set options for a static physical load address.
> > 
> > Generally, people should try to be compatible with the single kernel
> > image effort unless there's a really compelling reason not to.
> 
> Well, all the embedded kernels we use are always custom built and
> minimized for the target. So as long as there are options to minimize
> the kernel size/increase performance by taking out the relocation
> stuff, we are going to use them.
> 
> All our boards on PPC and ARM use DT kernels now, and we try to flow
> back all the generic stuff as best we can. As someone that makes
> custom boards, I really like DT, it makes things much easier :)
> 
> > Wouldn't your firmware be incapable of loading a multiplatform kernel?
> 
> Well, no, it boots ELFs, so it can boot anything, with any memory
> layout. A 2nd stage loader would be required to boot standard kernels,
> that loader would be an ELF with 1 section for the 2nd stage, 1
> section for the zImage and 1 section for the initrd, with proper load
> headers.

Don't you already have to treat Linux as a special case though?  How
do you know where to load ATAGs, DT and/or initramfs, and how to
initalise the registers?  None of that is part of any ELF specification,
and would be inappropriate if you boot any non-linux images.

> Creating such a system is a lot of annoyance, so we never have - it is
> *so much* easier to just boot vmlinux ELF directly.
> 
> > Well, that was a bit of a pedantic point I admit, but there are
> > conflicting definitions of what "virtual address" really means in these
> > situations.  The original SYSV ABI spec explicitly specifies that
> > e_entry is a virtual address, but is also rather vague about how the
> > paddr fields should be interpreted.
> 
> Granted the spec is vauge, but convention for loaders seems to be that
> it is a physical address these days.
> 
> It could be a virtual address, and the loader could translate it by
> looking at the phdrs, but I don't see any other arches doing that?

You would just give .head.text a virtual address matching its load
address.  But as you say, no other arches bother with this, and it's
not obviously worthwhile.

> > All that AT(ADDR(blah) - LOAD_OFFSET) stuff is cumbersome, but if it's
> > at least consistent with other architectures then it may not such a
> > disaster.  It's not universal though: less than 50% of the arches in
> > the kernel currently seem to use this.
> 
> I agree it is not nice, but I once did try to make PHDRS work as you
> described, but was never successful. IIRC there were serious linker
> bugs) As you note the AT method is consistent with other arches, and
> the generic vmlinux.lds.h

A quick experiment shows that 

	PHDRS {
		kernel PT_LOAD AT(PHYS_OFFSET + LOAD_OFFSET);
	}

	/* ... */

	SECTIONS {
		.head.text {
			/* ... */
		} :kernel

		/* ... */
	}

can produce a sensible-looking vmlinux at least with my version of the
tools.

As you observe, GNU ld behaviour in this area tends to be rather patchily
specified, buggy or both.  That does argue in favour of reusing the
same techniques already used for other arches, though.


A question does occur to me: do your changes work with XIP_KERNEL?
I'm not very familiar with XIP_KERNEL myself, so I'm currently not
clear on whether there's an impact here.

Beyond this, I think the approach doesn't look unreasonable.

> > > Yes, if you use more loader stages then the load headers are ignored.
> > > Our boot loaders on our boards boot straight ELF vmlinux.gz so they
> > > need correct load headers.
>  
> > If your image is compressed anyway though, why are you not using zImage?
> 
> We store the kernel in a CRAMFS, the loader pulls it out and
> decompresses it, processes the ELF sections 'on the fly' and jumps to
> it. Using zImage would result in double-decompression, and reallly has
> no benefits to us.

You store vmlinux.gz in a cramfs?  Is that a typo, or have you already
compressed the kernel twice?


Cheers
---Dave
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