Hi Paul,
First off, apologies for the flood of messages to you and everyone on-list =)
I'm seeing a few things that make for some weird and possibly unpredictable behavior here:
1. The rtc/timeoffset value is not preserved on migration, causing a migrated VM to revert back to dom0 time
2. The xenstore rtc/timeoffset is relative to the rtc_timeoffset specified in xl.cfg (they're not the same variable)
3. Likely due to #2, specifying rtc_timeoffset in the config isn't inserted into rtc/timeoffset on create
As an example:
1. Boot windows 2012R2 on hostA, with rtc_timeoffset 0 in the config. Host and VM are set to PST. VM clock shows 3pm correctly
2. Set the VM to EST, clock is now 6pm and /vm/UUID/rtc/timeoffset is now 10800
3. Migrate the vm to hostB
4. Clock reverts back to incorrect 3pm (still on EST), and /vm/UUID/rtc/timeoffset is now null
5. Shutdown the VM completely, and recreate it with config option rtc_timeoffset 10800
6. VM boots up with correct 6pm for the configured EST time zone, and /vm/UUID/rtc/timeoffset is now null
7. Migrate to hostA, clock stays on correct timezone, rtc/timeoffset is still null
This behavior ends up effectively meaning I need to disallow migrations on any VM that has had a local time change, until I can do a restart of it to set rtc_timeoffset at creation time. It also makes the logic for handling that setting a little trickier, as I need to keep track of the two values and relative difference.
Ideally I think both xl.cfg rtc_timeoffset and xenstore rtc/timeoffset should be treated the same and handled as a single setting, but that's more open to opinion. The resetting of rtc/timeoffset to null on migration however looks like a bug.
Cheers,
Nathan
From: Nathan March [mailto:nathan@xxxxxx]
Sent: Monday, August 15, 2016 11:34 AM
To: 'Paul Durrant' <Paul.Durrant@xxxxxxxxxx>; 'win-pv-devel@xxxxxxxxxxxxxxxxxxxx' <win-pv-devel@xxxxxxxxxxxxxxxxxxxx>
Subject: RE: [win-pv-devel] rtc timeoffset not being set on TZ changes?
Tracked this down, the default xenstore permissions don't seem to allow qemu-dm to write to it
Name ID Mem VCPUs State Time(s)
nathanwin 61 8191 2 -b---- 13.6
nathanwin-dm 62 32 1 -b---- 5.5
rtc = "" . . . . . . . . . . . . . . . . . . . . . . . . . . (n0,r61)
timeoffset = "" . . . . . . . . . . . . . . . . . . . . . . (n0,r62)
Changing this to be writeable, makes it work.
- Nathan
