Re: [PATCH] access gdtdesc on segment 0 unconditionally (Re: [PATCH] i386-qemu port)

[Pavel Roskin]

On Mon, 2009-06-22 at 22:52 +0200, Robert Millan wrote:
> On Mon, Jun 22, 2009 at 03:39:03PM -0400, Pavel Roskin wrote:
> > On Mon, 2009-06-22 at 11:52 +0200, Robert Millan wrote: 
> > > > Since %cs is pointing to the code, it should be possible to point it to
> > > > gdtdesc.  They should be nearby.
> > > 
> > > It is nearby, but the address reference for `gdtdesc' is absolute, NOT
> > > relative to %cs.  Of course, when %cs is 0 that's no problem.  But in my
> > > case I can't set %cs to 0 because my code is above 0x10000.
> > 
> > I think we can remove ADDR32 from the command.  I tried that on i386-pc
> > and it works in qemu and on real hardware.  I don't see any need to use
> > a 32-bit address in the lgdt command.
> 
> This won't build.  I don't think it's possible to use relative addresses
> with this particular instruction.  "DATA32 lgdt %cs:gdtdesc" results in:
> 
>   boot_img-boot_i386_qemu_boot.o: In function `real_to_prot':
>   (.text+0x64): relocation truncated to fit: R_386_16 against `.text'

My bad, I only checked it for i386-pc.  I see this error for
ieee1275-i386.

I believe it happens because we relocate the output too high in memory,
so we cannot relocate a 16-bit address there.

> What's the problem with removing %cs?  It's presence there is bogus.  It
> *seems* to indicate gdtdesc is a segment-relative reference, but in fact
> it's not, and it just happens to work because %cs was set to 0.

I just wanted to make sure we are not doing anything wrong.  I have a
feeling that somebody with a good knowledge of assembler tricks could
write the code to use %cs and a 16-bit address, but I don't know how to
do it.

Anyway, your code is correct and I have no objections.  Please commit.

-- 
Regards,
Pavel Roskin