Re: [Xen-devel] [PATCH] xen-unstable on HP dc7800 simply reboots

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 25/10/07 03:13, "John Byrne" <john.l.byrne@xxxxxx> wrote:

>> Once I identified the problem revision, I went back to the tip to try to
>> debug it. (Sorry for any ambiguity.) Taking out get_memory_map is
>> sufficient;  get_edd and video don't seem to cause any problems.
>> Removing the lidt changes nothing. Adding a "ret" after the .Lmem88 in
>> mem.S confirms that the problem is in the e820 call. I'm currently
>> trying to see if the descriptor table is getting corrupted during the
>> BIOS calls.
> 
> A lot of work to find a one-line fix. There is no sign of any corruption
> in the GDT, but you do need to reload the GDT before transitioning back
> to real mode. I am asking a BIOS person might require this, but, in the
> meantime, I cannot see how this patch will cause trouble on any other
> system and it seems to fix mine. I am just a little uncomfortable
> because I don't really understand why it is required.

This is a very reasonable thing to do actually, if for no other reason than
all bootloaders appear to do it every time they enter protected mode.
There's safety in numbers!

We should probably frob the a20 gate on entry/exit to real mode too, for the
truly authentic real-mode experience. Nothing's yet broken due to lack of it
though.

 -- Keir



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