What is machine address?

What is machine address?

[Chengyuan Li]

hi,

I find the words "machine->physical mapping" in Xen's source code.
What's the "machine address" ? And what's the difference between
machine address and physical address?


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Re: What is machine address?

[Steven Hand]

>I find the words "machine->physical mapping" in Xen's source code.
>What's the "machine address" ? And what's the difference between
>machine address and physical address?

Machine address[*] = address of a piece of RAM (starting at 0 and going
up to the amount of memory you have in your actual machine). 

Physical address = address of a piece of memory used by a guest operating
system (starting at 0 and going up to the amount of memory you have in 
that particular virtual machine). 

Simplified[+] example: your PC has 128Mb of memory. You run 2 virtual 
machines (domain 0 and domain 1) each with 64Mb of memory. 
For the guest OS in domain 0, physical addresses = machine addresses. 
For the guest OS in domain 1, physical address X = machine address 64Mb+X

hope this helps, 

cheers,

S.


[*] I'm ignoring non RAM machine adresses (e.g. bus addresses) here 
    for simplicitiy. 

[+] in reality (a) Xen takes some memory so no Guest actually has 
    identical machine and physical addresses (b) the mappings aren't
    necessarily as simple as that given above - in general any physical
    page in any guest OS can map to any machine page. This is what the 
    machine-to-phys and phys-to-machine tables are all about. 





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Re: What is machine address?

[Ronald G. Minnich]

MA = machine address
PA = physical address
VA = virtual
On Thu, 16 Sep 2004, Steven Hand wrote:

> Physical address = address of a piece of memory used by a guest operating
> system (starting at 0 and going up to the amount of memory you have in 
> that particular virtual machine). 

On real machines, PAs == the real hardware. 

On Xen domains, PAs == not real hardware. 
Xen actually virtualize physical addresses, so although the PA range in an 
OS may be linear and contiguous, the underlying MAs are not. 

At some point I want to fix up Plan 9 to ignore PAs entirely and just VA 
and MA, but that's for later. The concept of a PA has no real meaning 
anyway at this point, so I see no reason to deal with it.

ron


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Re: What is machine address?

[Kip Macy]

How does Plan 9 efficiently handle a large number fragmented memory
ranges? FreeBSD can do it, but not well enough to eliminate the PA/MA
dichotomy?

			-Kip

On Thu, 16 Sep 2004, Ronald G. Minnich wrote:

>
> MA = machine address
> PA = physical address
> VA = virtual
> On Thu, 16 Sep 2004, Steven Hand wrote:
>
> > Physical address = address of a piece of memory used by a guest operating
> > system (starting at 0 and going up to the amount of memory you have in
> > that particular virtual machine).
>
> On real machines, PAs == the real hardware.
>
> On Xen domains, PAs == not real hardware.
> Xen actually virtualize physical addresses, so although the PA range in an
> OS may be linear and contiguous, the underlying MAs are not.
>
> At some point I want to fix up Plan 9 to ignore PAs entirely and just VA
> and MA, but that's for later. The concept of a PA has no real meaning
> anyway at this point, so I see no reason to deal with it.
>
> ron
>
>
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>


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Re: What is machine address?

[Ronald G. Minnich]

On Thu, 16 Sep 2004, Kip Macy wrote:

> How does Plan 9 efficiently handle a large number fragmented memory
> ranges? FreeBSD can do it, but not well enough to eliminate the PA/MA
> dichotomy?

No, what I mean here is that Plan 9 would run with virtually contiguous
ranges but physically discontiguous ranges. I can't seen any reason it
won't work (yet). 

But hell, I have to get my corrupted receive ethernet packets fixed first 
:-) :-(

ron


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