Re: [GIT PULL] x86 setup: correct booting on 486 (revised)

From: H. Peter Anvin
Date: Mon Nov 05 2007 - 16:29:39 EST

Eric W. Biederman wrote:

I'm just saying be liberal in what you accept and conservative in what
you send.


Making the entire process well defined is useful so things don't break
unnecessarily, and the maintainers of the pieces of software that use
the interface know what they can reliably get away with and what is
just luck.

Currently using the 32-bit entry point reliably requires:
%cs to be set.
%esi to be set.
%ebx be set to 0.
%gdt to be set and have:
0x10 a 32bit 4G code segment with base of 0
0x18 a 32bit 4G data segment with base of 0

Actually, I suspect the currently code will handle %ebx with any value, but some older kernels might not handle that correctly.

With the latest generation of the boot protocol if KEEP_SEGMENTS
is set then it is only required that the data segments %ds, %es, %fs,
%gs and %ss be initialized to a valid value.

I have no problem with code providing more then what is required
above, and in fact I think it is likely a good thing.

For future expansion of the protocol things will go easiest if
we don't add additional requirements to the list above, as that
is all that I think all current boot loaders provide.

Specifying now that unused GPRs should be zeroed will allow for changes if and when we need it. It's an easy requirement to fulfill, so boot loader authors can put it through the pipe now. Then, if we find ourselves in a corner in the future, we have a possible out.

Anyway this is getting off topic. So far the changes to pmjump.S
look to be going well.

Thanks. I just pushed two more patches to the git tree; one to do the paranoia thing, and one to initialize LDTR and TR; the latter is for the benefit of Intel VT and is not required for correctness, but it should be able to speed up booting slightly on VT-based hardware.


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