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Re: [Xen-devel] Hypercall interface changes for PAE

On 31 May 2005, at 21:38, Gerd Knorr wrote:

I'd expect with a identical interface we'll only have to add a
PAE-enabled domain builder to boot domU (there are multiple
already anyway, right?), whereas with different interfaces the
split goes down to the shared lib which provides the hypercall
interface to the tools.

I don't think we want pae/non-pae builder made visible to higher-level tools. We can hide the auto-switch between builders within libxc itself. The same goes for domain save/restore code as well (although there is potentially scope for more binary-code sharing between pae/non-pae in this case).

Point is I expect it being much easier to switch at runtime
between pae and non-pae in the tools when the hypercall
interface is identical.  I might be wrong on this though.

The code that would be affected by an interface difference is precisely that code which manipulates page tables, and so is already broken by the different pagetable format. The only way unmodified non-pae code could possibly be made to work is by using shadow page tables. In that case we would hook off the call to e.g., do_mmu_update() very early anyway (off into shadow code). Hardly different really from jumping in the first place at a different hypercall function with different prototype: this latter would arguably be cleaner and less cluttered, as well as easily allowing us to support non-pae hypercall interface within pae xen.

I really think that creating this interface inconsistency is not something to be worried about.

 -- Keir

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