Messages in this thread | | | Date | Mon, 05 Nov 2007 13:06:58 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [GIT PULL] x86 setup: correct booting on 486 (revised) |
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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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