"H. Peter Anvin" <[email protected]> writes:
> Eric W. Biederman wrote:
>>
>> We have real mystery historical cases in Gujin and ELILO. So it
>> probably makes sense at this point to force a gdt reload if we can and
>> otherwise require all of the segments %ds, %es, %fs, %gs to be loaded
>> with a valid segment, that we can reach everything we need to touch
>> from.
>>
>
> I think we should avoid using %fs and %gs (no use for them, anyway); but
> I think it's a decent expectation to have %cs, %ds, %es, and %ss set up.
Yes we have no need for %fs and %gs. Lumping them in with the rest is
just extra helpfulness.
Basically be conservative in what you send (all segments).
Be liberal in what you accept (%cs,%ds,%es,%ss) and less if
we can get away with it.
> Gujin seems to have a near-zero user community, so if they have to rev
> their code it wouldn't be a big deal (the author keeps trying to push
> some crack-smoking "Gujin native" patches into the kernel, too),
> breaking ELILO would hurt anyone using Intel Macs.
I'm thinking we just make the code start.
startup_32:
movl %cs, %eax
testl $3, %eax
jnz 1f
lgdt boot_gdt_descr - __PAGE_OFFSET
movl $(__BOOT_DS),%eax
movl %eax,%ds
movl %eax,%es
movl %eax,%fs
movl %eax,%gs
movl %eax,%ss
1:
But that won't work if we want to support relocatability.
Because we can't load a gdt if we don't know where we are.
To find out where we are we need %ss and %ds, at which point
we might as well assume we have %es to.
I think that will work in the elilo case but we can't reload them.
As silly elilo loaded a new gdt but not it's segments...
So be it then. The next rev of the boot protocol gets to be partially
incompatible, and we just assume that %cs, %ds, %es, %ss meet our
basic requirements. I'm pretty certain from what I saw only Gujin
is going to suffer :(
Yep it is long past time that we document what needs to happen for
the 32bit entry point of the linux kernel.
Eric
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