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Re: [Xen-users] Quest for a better FAQ

  • To: "Henning Sprang" <henning_sprang@xxxxxx>
  • From: Derek <xen@xxxxxxxxxxxxxxxx>
  • Date: Tue, 10 Jul 2007 10:06:17 -0600
  • Cc: Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 10 Jul 2007 09:04:10 -0700
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  • List-id: Xen user discussion <xen-users.lists.xensource.com>


You've probably already got these FAQs on your list, but just in case,
here are questions I have either asked myself, or had to look up
elsewhere.  Not sure if they are all frequently asked, but they all
ought to be documented somewhere.

Which permutations of bittedness (64/32/32PAE-bit) and type of
virtualization (PV/HVM) domU work under each bittedness of dom0 ?

In the case of permutations which don't work, some information about
whether it's a fundamental limitation of the architecture, or whether
it's just an implementation thing likely to be temporary?

What is this PAE?  (If that's beyond the scope of xen documentation,
then perhaps the answer could be a URL pointing readers elsewhere).

How do I tell if my existing CPU/motherboard/chipset is VT compatible?

How do I choose a new CPU/motherboard/chipset that is known to be VT compatible?

Can I tunnel or otherwise map a particular PCI device to a particular
HVM domU?  Ditto for PV domU?

Can I tunnel or otherwise map a particular USB device to a particular
HVM domU?  Ditto for PV domU?

What are my options, if any, for running accelerated graphics
applications (e.g. Google Earth under Windows) within a domU?

Can I migrate a hardware-direct OS installation to run as an HVM domU?
Ditto for PV?  What tools, if any, exist to help with this?

How do I send ctrl-alt-del to a domU?  How do I change a removable
disk mapped to a domU?  ( & other questions about functionality
provided by the quemu-dm menu.)

I can't get into the qemu-dm menu.  Why?
Answer:  The keystrokes (ctrl+alt+something_i_forget) has to use the
_left_ ctrl and alt key.  If your keyboard only has the right one
(such as a Datahand keyboard), you are out of luck.

Can I use an off-the rack Linux kernel for DomU?  For PV dom0?  What
is the status of getting Xen patches into the off-the-rack kernel
source, so that they cease to be "special"?

Exactly which patches are applied to an off-the-rack kernel to make a
Xen kernel?  If I want to do the patching myself, where do I download
the patches?  Any special instructions?  [This would be useful for
people who want/need to experiment with different kernel revs than the
ones provided because, for example, of newer hardware support or bug

My mouse behaves horribly under HVM windows.  Why?
(Answer: need usbdevice="tablet" in config file.)

Xen startup messages scroll by too fast for me to read.  How do I know
what errors there were?
(Note:  xm dmesg is only a partial solution, since the system may not
boot up.  Slowing it by sending messages to serial doesn't work on
motherboards without a serial port e.g. HP d4600y.  So, I don't think
there's a really good answer to this question until someone adds a
"slowmedown" option to the xen grub command line options).

What is the role of qemu?  Where does it fit into the big picture?

If I think of others, I'll post them here.


On 7/10/07, Henning Sprang <henning_sprang@xxxxxx> wrote:
Dylan Martin wrote:
> I'm working on making a better xen faq.  Hopefully I can use it to
> replace the one we now have on the wiki.

Please, don't start a new one, but improve the one that is there!

> I'd love to know what people think really are the most frequently
> asked questions?

Hard to say ad-hoc. I think your start looking at the archives is a good

Add these you think which are not covered in the FAQ, and remove these
which are not up to date anymore. (Answers which are still valid but
don't seem frequently asked anymore should stay - they might just not be
frequently asked on the list because they are on the FAQ and some people
read it.

Apart from that, a lot problems I see occuring and reoccuring with Xen,
and coming as questions to the list are either:

- very strange behaviour of Xen under very special circumstances, like
specific (broken) hardware, and very specific setups, together with
quite esoteric and hard to debug errors occuring.
There are so many versions of Xen out there, and every distribution has
their own specific set of patches to Xen and the Xen Linux Kernel(and
thesde all are combined with gazillions of different hardware setups),
that it's hard to catch all these problems.

- bad error reporting and logging. This is not a documentation problem.
I did not run into these problems for a while because I have my fixed
setup, but there was times when I found it really bad how errors are
reported in Xen:
 - very first, the logfile where you _might_ find some useful
information in ist the one with "debug" in it's name - which is opposite
to usual behaviour. Normally, in debug i'd expect strange stuff only
developers understand, and in the normal log, stuff useful for users is
saved. Xen is the other way around.
 - bad error checking and reporting in the userspace tools. Like, for
example, if you don't set qemu_dm right in the config file, instead of
getting a decent warning, one just gets a Qemu screen which stays blank.
There was some other common misconfiguration that occured sometimes, and
resulted in a totally esoteric error message - I forgot it sadly. There
might be more of them.

- people who don't read the FAQ, user manual(admittedly outdated and
missing a lot of important things, still gives a very good overview of
what xen is about), and fail to search mailing lists. Sadly, you won't
catch these with a FAQ, and my attempts to set up a list netiquette
where not recognized by XenSource. even if one employee of them told me
he wanted to get the netiquette onto the List subscription page.

my 2 cents.

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