Xen users - Mar 2006 - Slightly OT: OS Resource Question

In principle, can a running Linux kernel recognize the addition of new
physical hardware resources such as CPUs, memory, or PCI devices?  Strictly
speaking, I''m not asking just about a running Xen instance -- but about
a
standalone, booted OS.
 
-M.  


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

________________________________

	From: xen-users-bounces@lists.xensource.com
[mailto:xen-users-bounces@lists.xensource.com] On Behalf Of Mark
Hargrove
	Sent: 08 March 2006 00:00
	To: xen-users@lists.xensource.com
	Subject: [Xen-users] Slightly OT: OS Resource Question
	
	
	In principle, can a running Linux kernel recognize the addition
of new physical hardware resources such as CPUs, memory, or PCI devices?
Strictly speaking, I''m not asking just about a running Xen instance --
but about a standalone, booted OS.
	 
	-M.   

Mark, 
 
I''m not an expert on this subject, but I beleive at the moment only by
kernel directly supported hotpluggable devices are PCI[xe] devices, USB
devices and hard-disks (SCSI, not sure about IDE). That is for a running
Linux kernel, and I don''t think it matters if it''s on top of
Xen or not.
 
Xen allows some trickery that can allow you to make more or less number
of CPU''s available to the guest by assigning more virtual
CPU''s than
actual CPU''s to the guest, and later on move the guest to more or fewer
real CPU''s. [That is, if I''ve understood things correctly]. 
 
There is the LHCS (Linux Hotplug CPU Support):
http://sourceforge.net/projects/lhcs/
This allows the capability of hotplugging CPU''s to the Linux kernel -
but it''s not a part of the kernel itself, it''s an external
patch.
 
There''s also a patch (which seems both old, kernel 2.6.7 and perhaps
incomplete - particularly check out the Known issues description).
http://people.valinux.co.jp/~iwamoto/mh.html
And the patch is just disabling/enabling the memory itself, not allowing
it to be physically removed/inserted.
 
--
Mats


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

> In principle, can a running Linux kernel recognize the addition of new
> physical hardware resources such as CPUs, memory, or PCI devices?  Strictly
> speaking, I''m not asking just about a running Xen instance -- but
about a
> standalone, booted OS.
Yes, it can - if the hardware supports it and the kernel you use has drivers 
for doing so.

The support for these things varies depending on what resource is involved.  
e.g. hotplugging memory is hard because some of the memory could contain 
kernel data structures, be pinned for IO operations, etc...

Under Xen you can hot(un)plug virtual CPUs to a virtual machine, and 
dynamically change the memory allocation (although it''s not strictly
hotplug,
the effect is similar).

On x86 you can hotplug CPUs to/from the kernel (I think???) but the hardware 
doesn''t actually allow them to be swapped whilst the machine is
running.  On
better hardware I believe this is possible.

HTH,
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!

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users