Does a PAE domU work with non-PAE dom0?

[Mark Weinem-3]

Hi,

a PAE domU will not work on a non-PAE dom0 - is that correct for a 32bit
system? Or does it depend on the used version of Xen?


Best regards, Mark Weinem
 
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Mark Weinem
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[Mark Williamson]

> a PAE domU will not work on a non-PAE dom0 - is that correct for a 32bit
> system? Or does it depend on the used version of Xen?

On 32-bit Xen all the PV guests have to match Xen.  So if Xen is PAE then dom0
and all domUs must be PAE.  If Xen is non-PAE then dom0 and all domUs must be
non-PAE.  You can't mix PAE and non-PAE.

On 64-bit Xen, PV guests (including dom0) can be 64-bit or 32-bit PAE.  32-bit
non-PAE PV guests won't run.

HVM guests can have any memory mode "equal or less than" the Xen memory mode.  
e.g. 32-bit non-PAE only on 32-bit non-PAE Xen.  32-bit PAE or non-PAE on PAE
Xen.  32-bit non-PAE, 32-bit PAE, 64-bit on 64-bit Xen.

Hope that helps,
Cheers,
Mark

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[James Harper]

>
> On 64-bit Xen, PV guests (including dom0) can be 64-bit or 32-bit PAE.
> 32-bit
> non-PAE PV guests won't run.
>
> HVM guests can have any memory mode "equal or less than" the Xen
memory
> mode.
> e.g. 32-bit non-PAE only on 32-bit non-PAE Xen.  32-bit PAE or non-PAE
on
> PAE
> Xen.  32-bit non-PAE, 32-bit PAE, 64-bit on 64-bit Xen.

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?

Thanks

James

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[Mark Williamson]

> 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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