Re: [Xen-devel] question about the meaning of memory auto-translate and paravirtual and no pseudophysical overlay

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

Nb. I'm focussing on x86 (and x86_32 where appropriate) here and have been in 
the rest of the thread (unless otherwise specified)...  other architectures 
deal with pseudophysical addressses differently.

> Mark Williamson 写道:
> >> if there is no pseudophysical addresses,a physical host computer could
> >> only paravirtualise one VM,is it right?
> >
> > No, AFAIK pseudophysical addresses are mostly there for the convenience
> > of the guest.  Xen has some support for them so that guests can use them
> > more efficiently but that's not strictly necessary.  In principle, they
> > could be eliminated from Xen entirely (would require modifying the PV
> > guests to manage the pseudophys abstraction themselves).
>
> Is there this kind paravirtual os for xen at present?
> or in future,will this kind paravirtual os come out?
> and what is the advantages for this kind of os?

There's two aspects to this answer, I guess:

1) Will guests which maintain their own pseudophysical abstraction rather than 
using Xen's be written?  /  Will Xen's pseudophysical support be removed?

This isn't likely for the timebeing; Xen needs to incorporate the 
pseudophysical support it has for backwards-compatibility purposes and given 
that there's no reason for guests not to use it.

2) Will guests that don't use a pseudophysical abstraction at all be written?

I don't know if anyone has written / ported an OS that does this...  I 
wouldn't expect to see any general purpose OSes using this for a while - they 
typically seem to need the pseudophysical abstraction to keep their generic 
memory management code happy.

More minimal, special purposes OSes (especially if targetted to Xen) might be 
able to do away with pseudophysical addresses entirely and just use virtual 
and machine addresses.

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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