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Re: [Qemu-devel] [ipxe-devel] Big real mode use in ipxe
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] [ipxe-devel] Big real mode use in ipxe |
Date: |
Sun, 19 Aug 2012 18:42:08 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120717 Thunderbird/14.0 |
On 08/19/2012 06:34 PM, Michael Brown wrote:
> On Sunday 19 Aug 2012 16:07:05 Avi Kivity wrote:
>> Which is exactly what happens here. My understanding of big real mode is
>> that to achieve a segment limit != 0xffff, you must go into 32-bit
>> protected mode, load a segment with a larger limit, and return into real
>> mode without touching the segment. The next load of the segment will
>> reset the limit to 0xffff.
>
> Not quite. You can't "return into real mode without touching the segment",
> since part of the process of returning to real mode is to reload the segment
> registers with real-mode values, and this happens _after_ setting CR0.PE=0.
>
> Whenever CR0.PE=0, loading a segment register with value N will load the
> literal value (N<<4) into the base address for that segment, without changing
> the limit. This is the trick that allows flat real mode (aka big real mode)
> to
> work; the limit remains at 4G even after loading the segment register with a
> real-mode value.
So I see, from looking at the Xen source. I'll also double-check with
bochs. Looks like I'll need to fix kvm not to reset the segment limit
when reloading a segment in real mode.
>
>> (and that seabios needs changes to either work in
>> big real mode, or to put the processor back into big real mode after
>> returning from a PMM service.
>
> If seabios switches into protected mode when performing a PMM service, then
> it
> _must_ leave the segment limits at 4G when returning to real mode. To do
> otherwise will violate the PMM spec, and will break conforming clients such
> as
> iPXE.
This probably works, since iPXE works on kvm on AMD and on Intel
processors with "unrestricted guest" support.
--
error compiling committee.c: too many arguments to function