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

[Mark Williamson <mark.williamson@cl.cam.ac.uk>]

> * This scheme provides consistent function and variable names even when
> * different guests are running in different memory-management modes.
> * 1. A guest running in auto-translated mode (e.g., 
> shadow_mode_translate())
> *    will have gpfn == gmfn and gmfn != mfn.
> * 2. A paravirtualised x86 guest will have gpfn != gmfn and gmfn == mfn.
> * 3. A paravirtualised guest with no pseudophysical overlay will have
> *    gpfn == gpmfn == mfn.
> *
> 
> what is the meaning about auto-translated mode,is it not the same with 
> paravirtualised one ?

An auto-translated paravirtualised guest could be Xen-aware in its
memory management code but Xen would be hiding the underlying memory
layout from it.  This is not the normal case - I'm not actually sure
it's supported these days, although it has worked in the past.

Essentially, though and autotranslated guest doesn't need to have any
awareness of the machine memory layout, it can just pretend its got all
the memory.

> does the auto-translated mode work for hvm and shadow mode? how does it 
> work?

HVM also has a version of autotranslate mode (does it have another name
for HVM?  I'm not sure...) - HVM guests always expect that they own
*all* the memory in the machine, so they need to have the real memory
layout concealed from them.  This is done by shadowing the guest's
pagetables so that a gmfn->mfn translation can be applied to them before
inserting them into the real pagetables.

> and what is the mechanism about paravirtualised guest with no 
> pseudophysical overlay?

This would be a guest that didn't use the abstraction of pseudophysical
addresses at all; it would just use virtual and (real) machine
addresses.  I'm not sure if there are any guests that operate in this
way...  Certainly the major heavyweight OSes all use this technique when
paravirtualised.

HTH,
Cheers,
Mark