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Re: [Qemu-devel] [PATCH] move 'unsafe' to end of caching modes in help

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH] move 'unsafe' to end of caching modes in help
Date: Wed, 21 Jul 2010 22:58:33 +0100
User-agent: Mutt/1.4.1i

On Wed, Jul 21, 2010 at 04:45:46PM -0500, Anthony Liguori wrote:
> On 07/21/2010 04:32 PM, Daniel P. Berrange wrote:
> >On Wed, Jul 21, 2010 at 03:55:28PM -0500, Anthony Liguori wrote:
> >   
> >>On 07/21/2010 03:32 PM, Bruce Rogers wrote:
> >>     
> >>>Libvirt parses qemu help output to determine qemu features. In particular
> >>>  it probes for the following: "cache=writethrough|writeback|none". The
> >>>  addition of the unsafe cache mode was inserted within this string, as
> >>>  opposed to being added to the end, which impacted libvirt's probe.
> >>>  Unbreak libvirt by keeping the existing cache modes intact and add
> >>>  unsafe to the end.
> >>>
> >>>This problem only manifests itself if a caching mode is explicitly
> >>>specified in the libvirt xml, in which case older syntax for caching is
> >>>passed to qemu, which it  no longer understands.
> >>>
> >>>Signed-off-by: Bruce Rogers<address@hidden>
> >>>
> >>>       
> >>Errr, libvirt is still doing this?
> >>
> >>This comes up frequently and it's a real PITA.  Help text is not a
> >>feature probing interface.  This is a libvirt bug and it needs to be
> >>fixed in libvirt
> >>     
> >Of course, there is no viable alternative yet.
> Yes there is.  Use the version number.

The version number is not suitable, because features can be removed at
compile time and/or added via patch backports. The only reliable way is
to query the QEMU binary to ask it what it actually supports.

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