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Re: [Xen-users] Does a PAE domU work with non-PAE dom0?

To: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Does a PAE domU work with non-PAE dom0?
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 4 Dec 2007 04:52:29 +0000
Cc: Mark Weinem <mark.weinem@xxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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> Just out of interest, with a 64-bit hypervisor, which of the HVM options
> (32, 32p, 64) will give the lowest overhead? Or are they going to be
> more the same than they are different?
Hmmm. I suspect they're more the same than different... It *might* be that 
one or the other reduces the overhead of maintaining shadow pagetables. But 
I wouldn't be surprised if they came out fairly similar in terms of overhead.
Of course, if you use a 64-bit guest then the guest applications get the 
advantage or a larger virtual memory space and of extra registers, so they 
may benefit in performance because of that.
Amusingly for PV guests, I strongly suspect the overhead is lower when running 
32-bit PAE guests on 64-bit Xen than when running native 64-bit guests. That 
may well extend to dom0 too. Also, glibc's negative segment offset tricks 
would work for 32-bit guests under 64-bit Xen at full performance... Crazy 
stuff ;-)
Cheers,
Mark
-- 
Dave: Just a question. What use is a unicyle with no seat? And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!
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