Re: [Xen-devel] question about machine-to-physic table and phy-to-machine table

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

>  I read the code ,there are machine-to-physic table and
> physic-to-machine table
> there are machine address for hardward address ,physic address for
> guestos's view hardware and virtual address ,is it right?
>
> phy-to-machine table is a mapping for guestos's view hardware to real
> hardward ,is it right?
> I am confused about the meaning and function of  machine-to-physic address

* Machine addresses represent real RAM in the host.  The memory a guest owns 
will certainly not start at 0 and will not necessarily be contiguous - it 
might be in a number of chunks with big gaps between.

* (pseudo)physical addresses represent the memory the guest owns.  This 
address space starts at 0 and is contiguous.

* Virtual addresses are used by software running in the guest, and by the 
guest kernel.  They're translated by the host CPU into machine addresses so 
that it can access the correct RAM.

Guests use physical addresses as an abstraction: most operating system memory 
management code assumes that the RAM owned by the OS starts at 0 and is 
contiguous.  Because this is not the case for Machine addresses under Xen, 
most of the guest's code is "tricked" by giving it pseudophysical addresses 
that look like it expects memory to look.

The P2M and M2P tables record the relationship between pseudophysical page 
frames (which the core OS code uses) and machine page frames (which the host 
really uses).  The Xen "architecture" code within the guest OS uses these 
tables to manage the translation between pseudophysical and machine page 
frames so that the guest's page tables can be handled correctly.  For 
paravirtualised guests, page tables must contain machine addresses - these 
must be translated from the pseudophysical addresses used by core OS code.

Hope that helps clarify how this all fits together, tgh.

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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