Re: [Xen-devel] Multiple priviliged domains

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>]

> 1. What is the model for allocating processor time to Domain-0?  Based on 
> my read of the Xen docs to date, I would expect it to [at least be 
> intended to] have an unbounded priority share of the total processing 
> resources, with some attempt at allocating unprivileged-domain-specific 
> processing (e.g., handling I/O or memory allocation requests) to the 
> requesting unprivileged domain.  Along these lines, should there be a 
> parameterizable configuration file for Domain-0?

At the current time, there's no attempt to 'recharge' CPU time
domain 0 spends doing I/O on behalf of other domains back to the
domains.  It would actually be pretty tricky to do this
accounting within Linux.  Fortunately, if you've got decent
DMA-capable hardware with interrupt coalescing then domain0
typically doesn't use too much CPU anyhow.  Obviously if you're
using loop files for VBDs or other higher-level processing the
situation might be different.
 
> 2. Have there been discussions about allowing multiple simultaneous 
> privileged domains, among which the physical resources are split?  Or 
> perhaps "semiprivileged" domains -- for example, a domain that handles all 
> the I/O requests to a particular storage device, or alternatively handles 
> all the I/O requests for a particular class/subset of unprivileged 
> domains?  I envision a desire for a master control partition (with 
> priority resource allocation) that forms the root of a hierarchical domain 
> structure, under which one or more I/O partitions execute.  (I recall 
> reading about this sort of design in one of the older VMM papers, or 
> perhaps a recent Denali paper?)

It's quite possible to have multiple privileged domains owning
different bits of hardware -- see the paper on I/O on the
website. This can even be used to enable the system to recover
from crashes due to buggy device drivers without a reboot.

The control tools haven't always made this easy to set up as its
not something that many people do.

> 3. I don't seem to be able to create more than 8 VBDs. [I am using 
> xen-2.0.1-src.tgz.]  While trying to start 5 domains, each of which had 
> two "scsi" disks (/ and swap), I discovered that the 5th domain wouldn't 
> start until I removed the swap disks from both the 4th and 5th domains' 
> configuration files -- i.e., not exceeding 8 VBDs total.  A cursory search 
> through the code didn't reveal any relevant #define's, and I didn't see 
> anything about this in the Xen docs, so before I look further I thought 
> I'd ask the list if this is a known limit.  (If so, I am surprised the VBD 
> structures aren't allocated dynamically to prevent this?)  A search of the 
> mailing list revealed a message from Ian on 2004-03-10 stating "Mark wrote 
> a pretty good readme on VBDs and put it in the tree", but I couldn't find 
> the readme; is it still part of the tree?

Are you using loop files? If so, Linux has a statically compiled
allocation, set to 8 by default. Fixing this and submitting the
patch to lkml would a good thing...

I don't believe the VBD code has any such limitations. (We tend
to use LVM rather than loop files).

> 4. On a loosely related note, what regression tests are used by the Xen 
> developers?  (I.e., what should be run before generating patches?)

ltp, ttcp and lmbench are useful. We run osdb/postgresql,
postmark and specweb/apache from time to time too. 

We have a nightly regression test procedure, but its rather old
now, being based on RH7.2. As such, it misses some of the
excitement caused by NPTL etc. It would be very useful to get an
automated regression test setup on something more modern, with as
many apps as possible.

Ian

> -- 
> Dr. John Linwood Griffin
> Research Staff Member, Secure Systems Department
> IBM T.J. Watson Research Center, Hawthorne, New York, USA
> JLG at us.ibm.com, http://www.research.ibm.com/people/j/jlg/
> 
> 
> 
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