Re: [GIT PULL] x86 setup: correct booting on 486 (revised)

[H. Peter Anvin]

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
> > Nailing down the interface as hard as possible is a good idea, to
> > avoid tying your hands for the future. 
>
> Erm, I guess I see what you mean, but it comes to the effect of tying
> your hands now in a specific way, rather than having them tied in an
> unknown way later on...

Sort of.  The problem is you can frequently not

> But I hadn't noticed the 32-bit boot protocol spec go in.  Unfortunately
> it isn't useful for booting a pv Xen guest; I just mailed my comments. 
> I hope we can iterate this to something more generally useful before
> getting too wedded to the current protocol.

I'm not so sure about that.  Xen PV is rather fundamentally a different 
beast, hence the platform field recently added to the protocol.

>    2. Also, Xen reserves the last chunk of the GDT for its own
>       descriptors, and by default boots the guest with the segment
>       registers preloaded with selectors pointing to flat 4G descriptors
>       in this range.  These segments are not a full 4G, since it uses
>       segment limits to protect the hypervisor from guests.  The limits
>       are as large as they can possibly be.  I like to see the boot
>       protocol require that all segments registers are preloaded with
>       large flat 32-bit descriptors, but not require a specific selector
>       value.  I'd happy to require the GDT to be active, so the segment
>       registers can be reloaded with their current values, until the
>       kernel establishes its own GDT.


This is addressed by the "don't reload segments" bit in LOADFLAGS.

	-hpa
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