Xen devel - Mar 2007 - query on pinning mechanism

query on pinning mechanism

hi all,

After going through the code, this is what i make out of pinning in Xen.

Please correct me if i am wrong.

A page can have mutually exclusive roles in the Xen depending on its current
use.

A page can have its reference count incremented or type count incremented
depending on its usage to avoid returning it to the pool of free pages and to
change its page type.

At times guest wants some of its page tables to exist even after a preemption.So
that page tables exist it must not loose its reference count.Pinning makes sure
that the page has a usage of 1 atleast for that particular page and avoids that
page to be assigned to free pool or reclaimed for a different type. 

Am i right?

Another doubt, Xen guest domains cannot swap pseduo physical memory.Am i right?

Can xen lxr repository on xensource be changed to a more organised way wrt
versions ?

linux-2.6.xx-xen etc are there but xen directory is same for all of them.

Can the directories be made version wise with all included sources as they
appear on untarring?

Thank you.

--pradeep

-----------------------

Pradeep Singh

Member Technical Staff

Core Technology Divison

53, Greams Road.

HCL Technologies, Chennai


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>  After going through the code, this is what i make out of pinning in Xen.
>  Please correct me if i am wrong.
>
>  A page can have mutually exclusive roles in the Xen depending on its
> current use. A page can have its reference count incremented or type count
> incremented depending on its usage to avoid returning it to the pool of
> free pages and to change its page type.
As I recall, pinning increments the type count.  I''m not sure that 
incrementing the refcount would be necessary.  The code implementing this 
will be in Xen''s arch/x86/mm.c somewhere, I''d think.
>  At times guest wants some of its page tables to exist even after a
> preemption.So that page tables exist it must not loose its reference
> count.Pinning makes sure that the page has a usage of 1 atleast for that
> particular page and avoids that page to be assigned to free pool or
> reclaimed for a different type. 
>
>  Am i right?
Yes, you''re right (although you might want to clarify which count is 
incremented...  I *suspect* it''s just the type count but
haven''t actually
looked at this code for a while).
>  Another doubt, Xen guest domains cannot swap pseduo physical memory.Am i
> right?
Swap to disk?  Xen domains can do swapping of their own if they want,
it''s
just that Xen doesn''t perform any swapping on behalf of domains (unlike
other
hypervisors like e.g. VMware ESX which are able to overcommit the host 
machine memory and then swap the VMs transparently).

Cheers,
Mark
>  Can xen lxr repository on xensource be changed to a more organised way wrt
> versions ? linux-2.6.xx-xen etc are there but xen directory is same for all
> of them.
>
>  Can the directories be made version wise with all included sources as they
> appear on untarring?
>
>  Thank you.
>
>  --pradeep
>  -----------------------
>  Pradeep Singh
>  Member Technical Staff
>  Core Technology Divison
>  53, Greams Road.
>  HCL Technologies, Chennai
-- 
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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