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

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