Re: [Etherboot-developers] Mknbi is the misbehaving when reloc'd

[Marc S. <el...@bu...>]

On Sat, Sep 21, 2002 at 01:01:40PM -0600, Eric W Biederman wrote:
> There are two ways to do PIC on x86.  And I have played with and implemented
> both of them.
> 
> 1) Use -fPIC when you compile and actually change all of the absolute references
>    in your code to know about the current load address.  See memtest86-3.0
>    for an implemenation.  The more relocations the linker can resolve
>    at link time the better.
> 

I was daunted by the need to do the fixups.  I'm familiar with the
MIPS and 68K where PIC code is much simpler.

> 2) Modify the code to use a protected mode code and data segment with base != 0.
>    This is what I have done with etherboot.  If you need to refer to an absolute
>    memory location you need a virt_to_phys/phys_to_virt set of calls.  This
>    technique is still compatible with the existing
>    real_to_prot/prot_to_real stubs in mknbi.  Except for the expense
>    of modifying the code this method is not to bad.

Are you doing this in the version of etherboot you are debugging?

This occurred to me.  In fact, I thought that it might be simplest to
use other segment registers to make it something built-in at compile
time.  No performance penalty except for the extra segment prefix and
a possible segment register load.  It could be macro-ized for some
cases, though I suspect it would be necessary to perform a copy in
many cases.

Cheers.