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

To: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Hypercall interface changes for PAE
From: Gerd Knorr <kraxel@xxxxxxx>
Date: 01 Jun 2005 11:17:02 +0200
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"Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx> writes:

> > But it is greatly simplified, IIUC. If the hypercalls are 
> > binary compatible then you have only one set of hypercall 
> > interface functions and types, and switching between two 
> > *implementations* of pagetable-related stuff (only the things 
> > that actually need to be different) is quite straightforward.
> For the same reason that Linux doesn't support run-time switching
> between PAE and non-PAE kernels, doing the same on Xen is going to be an
> equivalent pain in the butt.

Fully agree on the xen kernel side.  The performance hit a runtime
switch would have likely is bigger than simply running PAE all the
time ;)

I don't thing the performance argument is that important for the xen
tools though.  Booting or migrating a domain is a rare event (when
compared to the page table manipulations the xen kernel has to do all
the time).

> The only way it can reasonably be done cleanly and with decent
> performance is double compilation of the relevant mm functions in Xen
> (and libxc too). In which case, having separate hypercall vectors makes
> most sense.

Well, I'd try to get away without double compilation for libxc.

But you guys know that part of the code much better than I do, so if
you think double compilation is the best way to deal with it, lets
take that route.



-mm seems unusually stable at present.
        -- akpm about 2.6.12-rc3-mm3

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