[SeaBIOS] [RFC PATCH v3 0/3] Add segment limit checks to emulator

Kevin O'Connor kevin at koconnor.net
Sun Jul 25 19:19:58 CEST 2010

On Sun, Jul 25, 2010 at 12:42:46PM -0400, Kevin O'Connor wrote:
> On Sun, Jul 25, 2010 at 11:55:47AM +0300, Avi Kivity wrote:
> > What conditions are needed to trigger this path?  This can't occur
> > under normal operation, since it will fail badly with kvm on Intel.
> It's called on every boot.  I've personally only tested kvm on amd,
> but I'd have to assume something must be allowing this to work on
> intel.

BTW, the transition16big code does:

        ljmpl $SEG32_MODE16BIG_CS, $(0xf0000 + 1f)

        // Disable protected mode
        movl %cr0, %eax
        andl $~CR0_PE, %eax
        movl %eax, %cr0

        // far jump to flush CPU queue after transition to real mode
        ljmpw $0xf000, $2f

Only the ljmpw is in big real mode with a code address >64K - the
"Disable protected mode" code is technically in 16bit protected mode.
I'm not sure if that helps explain why it works.


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