Xen users - Jan 2008 - Questions about fast clock in 64-bit linux hvm

Hello all,

I''ve got a couple of questions about an issue I''m seeing in
Linux HVM
guests on 64-bit AMD systems. The clock is running slightly fast, and it 
builds over time at different rates on different guests to become an issue 
in virtual server pods we''re testing. I upgraded to Xen 3.2.0 from
3.1.2
recently but the issue hasn''t improved so far. I have a couple of
specific
questions about the problem:

1) I see that there are updates to timer modes for different HVM guest 
OSes. There isn''t any documentation anywhere, but I tracked down the 
"timer_mode" xm domain config variable, and found the explanations of
the
different values in xen/include/public/hvm/params.h. It looks like the 
options are delay_for_missed_ticks (default, 0), no_delay_for_missed_ticks 
(1), no_missed_ticks_pending (2) and one_missed_tick_pending (3). I tested 
0 and 1 so far. I saw good results on one domain that hadn''t had much
skew
in the first place, but the second one I tested was still a second fast 
soon after I created it. Does anywone have information on what mode works 
best with 64-bit linux?

2) These hosts are already running ntpd, but it doesn''t seem to be
working.
Are there particular settings I can use or is there a problem with certain 
versions of ntpd that will prevent it from working correctly in HVM virtual 
OS instances? Using ntp to correct the problem isn''t really a fix for
the
issue, but it would be a workaround for the moment while I look for actual 
solutions.

3) In the meantime, if the other options don''t work, it looks like I
will
have to run a cronjob using hwclock to reset the system time to the 
"hardware clock," which seems to be the system time of the dom0. Is
this
the case, and if so can I just run ntpd on dom0 and sync the domU times 
periodically using hwclock? This is a gross, herrible hack on my part and 
I''d prefer not to use it, but would it be considered viable in any way?


If anyone has any information or experiences with problems similar to mine, 
I''d appreciate any input. I''ve been playing with this for some
time and
while it keeps looking like the issue is improving, it''s never quite
there
yet. I really want to push Xen for more non-production usage in my company, 
but I just need to iron out these few kinks beforehand. Thanks for any help 
you can give.

Brendan Wood
Responsys, Inc

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