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Re: [libvirt] [PATCHv2 0/3] Xen: Fix <clock> handling



Hello Eric,

On Monday 13 February 2012 08:51:41 Philipp Hahn wrote:
> On Thursday 09 February 2012 23:01:38 Eric Blake wrote:
...
> > But a better idea is to represent the XML in a way where the default
> > conversion makes sense, but where a user can explicitly change the XML
> > to get what they want.  I'm thinking:
> >
> > If offset is 'utc' or 'localtime', we add a new attribute 'adjustment'
> > ...
>
> I'll give your approch a try, since I think this solves the problem with my
> STRICT.

I encountered some problems while thinking more about your suggestions:

1. The problem here is that the libvirt XML parser (virDomainDefParseString()) 
is more or less driver agnostic and thus does not know, which variants of 
clock/@offset are supported by the driver: For example since for qemu the XML 
is used as the native format, some values are only checked when you run the 
domain, that is when the XML in converted to command-line arguments to kvm. 
So your extra @reset and @adjustment attributes need to be pased into "struct 
_virDomainClockDef".
I see no canonical location to then convert <clock offset='utc' 
adjustment='42'/> to <clock offset='variable' adjustment='42' basis='utc'/> 
for everything but Xen-HV≥3.1, so the check / conversion has to be done for 
every single driver.

2. Instead of implementing a second variant of offset='variable' it would be 
enough to just add an attribute reset='true' (or 
this_is_not_from_a_buggy_libvirt_and_I_really_need_the_reset='true') to 
dumpxml, when a driver really implements the reset semantics. As long as it's 
missing, libvirt can convert it to reset='false' in the Xen≥3.1 case. With an 
explicit reset='true' libvirt would return an error for Xen-HV≥3.1.

A more general solution would be to add a libvirt-version attribute to the XML 
data, which libvirt could use to implement a bug-compatibility-mode: If we're 
reading an old XML, treat offset='utc' as offset='variable', but if it's an 
XML from ≥0.9.10, return an error. (how to treat XML without an explicit 
version?)
Yes, I know that the libvirt XML is supposed to be backward compatible, but 
errors happen.

Sincerely
Philipp Hahn
-- 
Philipp Hahn           Open Source Software Engineer      hahn univention de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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