On Wed, 2012-01-11 at 10:20 +0000, Philipp Hahn wrote:

Hello,

I'm currently tracking a problem in libvirt regarding Xens handling of localtime and rtc_timeoffset. My current understanding (Xen-3.4.3 and Xen-4.1.2 under Linux) of Xend (the depcrecated Python one still used by libvirt) is as this: - for HV domains, the RTC gets setup to either UTC or localtime depending on "/domain/image/hvm/localtime" ± "/domain/image/hvm/rtc_offset".

Are those xenstore paths or a reference to a config file/sxp var? I don't see the string "rtc_offset" anywhere under tools in xen-unstable.

- if the OS of a domU changes its RTC, the rtc_offset gets adjusted and is saved in XenStore as "/vm/$UUID/rtc/timeoffset".

I think so, yes, qemu appears to do this.

- if the dom0 accesses its RTC, is accesses the real HW-RTC. - the Xen-Hypervisor initially read the HW-RTC to setup its Wallclock once, which is than used to simulate the domU RTCs. (The HW-RTC is otherwise only accessed on (ACPI-)Suspend and Resume, and with NTP-drift-correction from dom0). - on shut-down the rtc_offset is stored by Xend in the "/var/lib/xend/domains/$uuid/config.sxp" file in "/domain/image/hvm/rtc_timeoffset", from where it is loaded again on next start. - since PV domains don't have a RTC, they somehow(?) get either initialized to the localtime or UTC time depending on "/domain/image/linux/localtime".

They get PV time direct from the hypervisor which exposes the hypervisor's wallclock to guests via the shared info. It is always UTC. See http://xenbits.xen.org/docs/unstable/hypercall/include,public,xen.h.html#Str... in particular the wc_* fields. A pvops domU will only sample this at start of day and then will maintain free-running wallclock time itself from then on (it is recommended to run ntp within such domUs). Classic-Xen kernels by default will take wallclock time from the shared info for each gettimeofday but can be configured to be free-running (that is the "independent_wallclock" mode).

@xen: Did I figure out that correct?

@xen: Is there some documentation on the Xen-sxp domain configuration? For the Python based xen-xm format, I found (and updated) <http://wiki.xen.org/wiki/XenConfigurationFileOptions>, but for Xen-sxp I so far found no documentation, especially on what changed between xen-1, xen-2, xen-3.x, xen-4.x.

Xen-1 and -2 had different architectures and toolstacks IIRC so we can discount them I think. I'm afraid I don't know of any resource for the -3/4 sxp stuff.

@libvirt: Comparing Xend handling to <http://libvirt.org/formatdomain.html#elementsTime> the current translation done by libvirt looks wrong; I think is mandates back to the time when Xen supported only PV-domUs: libvirt translates the Xen configuration to "localtime" and "utc" ignoring the "rtc_offset", which exists for HV domains. For localtime=0 this translates to libvirts offset="variable"-case, but for localtime=1 there is no matching mapping in libvirt. Since for PV domains no rtc_timeoffset is tracked, there the mapping to "utc" and "localtime" looks right.

For libvirt there was a patch <http://www.redhat.com/archives/libvir-list/2009-January/msg00757.html> which added some special handling for "localtime" to be either placed in "/domain/localtime" or "/domain/image/{hvm,linux}/localtime". Xend from 3.4.3 und 4.1.2 seems to accept either one, but /domain/image/hvm/localtime is preferred and overwrites the first one. When reading back the configuration the setting is always returned as /domain/image/{hvm,linux}/localtime.

@John: Is there a case, where /domain/localtime is returned or is that key always translated to /domain/linux/{hvm,linux}/localtime? As you had a sun.com email address, was this some special case when using Xen with Solaris?

@libvirt: The attached patch (for 0.8.7) would change the implementation to match the following: 1. For Xen-PV-domUs, use clock/@offset='utc' and clock/@offset='localtime'. 2. For Xen-HV-domUs, use clock/@offset='variable'. 3. For backward compatibility with old libvirt-XML-files convert clock/@offset='utc' → (localtime 0)(rtc_timeoffset 0) and clock/@offset='localtime' → (localtime 1)(rtc_timeosset 0). On readback that will be returned as clock/@offset='variable'! 4. For Xen-HV-domUs with (localtime=1)(rtc_timeoffset≠0) print a warning that there is no mapping to libvirts XML. 5. Always put the (localtime)(rtc_offset)-SEXPRs in "(image ({linux,hvm})", since this is where Xend-3.4 and Xend-4.1 return them. I also checked Xen-3.2, where this is okay, but the I don't have any older versions of Xen available (and running), the I can't verify that it still works there.

Which leads me to a another question: Which versions of Xen are still supported by libvirt (and must be checked for regressions)? I don't want so actively remove the code for old Xen versions, but it gets harder and harder to maintain all those versions. So a statement like "Xen-3.x and Xen-4.y are actively supported by libvirt-0.a.b; older versions might still work (by accident ;-)"

Before I forward-port that change to 0.9.10 I'd like to get some comments. Thanks in advance.

Sincerely Philipp Hahn