RE: [Xen-users] domU machines hang when Hyperthreading enabled in BIOS

["Samuel Torton" <samuel@xxxxxxxxxx>]

Hi,


I found the solution (or workaround only ?) to prevent from hanging up xenU
machines on a xen0 + BIOS HT enabled.

I must say I had another issue with the previous I described yesterday (see
thread).
I couldn't set up the max number of CPUs for my xen0 machine.
Whatever the "(dom0-cpus)" parameter value in my /etc/xen/xend-config.sxp
config file, I always had the following result in the "xm list" command
line:
   > vn2:~# xm list
   > Name        ID Mem(MiB) VCPUs State  Time(s)
   > Domain-0    0      500     4 r-----   967.6
It was not possible for me to set up a max number value of CPUs on my dom0,
it was always set to the MAX (ie. 2 proc + HT enabled = 4 CPUs)

Thus, I added the following parameter in my GRUB/menus.lst config file:
"maxcpus=N"
example:
  module /vmlinuz-2.6-xen0 root=/dev/sda3 ro maxcpus=3 console=tty0
max_loop=32

Then I rebooted.
... and suddenly my both problems where solved:

-1- I can limit my number of CPUs to a maximum value for xen0 and xenU
machines;
-2- My xenU stopped hanging up on boot !
    (I must say it was not a kernel panic, but just a hang up... can't
explain it)


However, now I have some other inherent questions:

- I cannot set dynamically number of CPUs on my xen0 machine when it runs,
  ie. the command "xm vcpu-set 0 N" doesn't work !
  (whereas it works for xenU machines)
  --> is it normal ?
      is it possible in theory to change that number of CPUs on a running
xen0 ?

- What's the difference between (dom0-num-cpus) and (dom0-cpus) ?
  Which one is really correct ?
  Is it (dom0-num-cpus) for Xen2 and (dom0-cpus) for Xen3 ?

- Whatever the value I give to my "(dom0-cpus)" parameter in my
  /etc/xen/xend-config.sxp file, it's totally unuseful... I really don't
know
  what's the influence of this parameter to my machines ???


That's why I'm wondering whether I really found the solution to my previous
problem, or if it's only a workaround ?



Thank you,
Samuel






-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of Samuel Torton
Sent: Tuesday, January 24, 2006 12:10 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] domU machines hang when Hyperthreading enabled in
BIOS


Hi,

I can't get rid of an annoying domU machines hanging issue with
Hyperthreading enabled in my BIOS.

Hardware config: IBM x325 series
                 2 * CPUs Intel Xeon 3.06 Ghz

Xen version:     3.0.0
Kernel version:  2.6.12.6
Linux distrib:   Debian Sarge 3.1 r1

I run 1 domU machine on my dom0, here is my Xen config:
  . xen0:
   /etc/xen/xend-config.sxp : (dom0-num-cpus 0)
  . xenU:
   /etc/xen/myxenU.cfg      : vcpus = 2


- First case:  . BIOS Hyperthreading : DISABLED
               . xen0 kernel config  : SMT enabled
               . xenU kernel config  : SMT enabled or disabled (whatever)
  When I reboot the physical machine, and run the command "xm list" :
   . dom0 is visible and works fine
     --> I see 2 cpus.
   . domU is visible and works fine
     --> I see 2 cpus;
         I can console the xenU machine and work inside.

- Second case: . BIOS Hyperthreading : ENABLED
               . xen0 kernel config  : SMT enabled
               . xenU kernel config  : SMT enabled or disabled (whatever)
  --> When I reboot the physical machine, and run the command "xm list" :
   . dom0 is visible and works fine
      --> I see 4 cpus.
   . domU is visible and SEEMS to work fine (!)
      --> I see 2 cpus;
          If I console the xenU machine, I see the machine
          has booted but is hanged before finishing the boot !!! :-(

Would you have any idea, any clue ? Do you have any experience ?
  - Where to search in log files in order to detect the problem ?
  - Which kernel parameter would I have forgotten ?
  - I don't even know whether the problem comes from xen0 or xenU kernel
conf !
  - etc... ?

Thank you in advance...
... please, don't advise me I have to disable Hyperthreading in BIOS !;-)

Regards,
Samuel


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