[libvirt] [PATCH v2] qemu: Add option to enable/disable IOEventFD feature

[Daniel Veillard]

On Wed, May 18, 2011 at 04:07:30PM +0800, Daniel Veillard wrote:
> On Tue, May 17, 2011 at 03:56:11PM +0100, Daniel P. Berrange wrote:
> > On Tue, May 17, 2011 at 04:49:17PM +0200, Michal Privoznik wrote:
> > > This feature allows QEMU to achieve higher throughput, but is available
> > > only in recent versions. It is accessible via ioeventfd attribute
> > > with accepting values 'on', 'off'. Only experienced users needs to set
> > > this, because QEMU defaults to 'on', meaning higher performance.
> > > Translates into virtio-{blk|net}-pci.ioeventfd option.
> [...]
> > > +          <li>
> > > +           The optional <code>ioeventfd</code> attribute enables or disables
> > > +           IOEventFD feature for virtqueue notify. The value can be either
> > > +           'on' or 'off'.
> > > +            <span class="since">Since 0.9.2 (QEMU and KVM only)</span>
> > 
> > This is a qemu specific attribute name & description. IMHO we shouldn't
> > be exposing that directly. Who even knows what effect it actually has
> > on the guests...
> 
>   Agreed, what is the semantic of this flag, beside allowing to switch
> something in qemu ?

 Just to clarify my answer a bit, the problem here is that the patch
does not explain what the ioeventfd qemu flag does in practice and how
it influence the virtualization. To be able to provide a good API and
maintain it long term we need to be able to explain the semantic of
the API (be it a function of the library or part of the XML being used),
only then we can guarantee that there is no misunderstanding about what
it does, and also allow us to reuse it in case the same functionality
is provided by another hypervisor.
 So instead of explaining the option using terms from QEmu, let's
explain what it does in general terms and use those general terms to
model the API,

  I hope it's clearer :-)

   thanks !

Daniel

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