Re: [Xen-devel] [Patch] Fix for x86_64 boot failures due to bad segment setup for protected mode.

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 9/11/06 3:49 am, "Stephen C. Tweedie" <sct@xxxxxxxxxx> wrote:

> The fix is to save the 16-bit segments *always*, on entry to protected
> mode when %CR0(PE) is first set; and to clear the saved 16-bit segment
> and set the 32-bit variant in oldctx whenever a 32-bit segment
> descriptor is set during the transition to 32-bit CS.  Then, when we
> finally do the VMENTER, we will set up the VMCS from only the 32-bit
> segments, clearing the VMCS entries for segments that have not been
> assigned valid 32-bit segments yet.

So, after setting CR0.PE but before doing a jump to complete the transition
to protected mode, is the guest now running with zeroed data selectors but
with the old 'shadow segment state'? So it can still use its data segments
until the long jump? Not that I know whether any bootloader would actually
*want* to access data in that critical region....

 -- Keir



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