[libvirt] [Xen-devel] [BUG, PATCH-RFC] libvirt localtime and rtc_timeoffset handling in xen-sexpr/sxpr/sxp

[Ian Campbell]

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#Struct_shared_info
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