Re: [Xen-devel] [PATCH v3 5/5] libxl: add domain config parameter to force start of qemu

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Mar 24, 2016 at 08:28:45PM +0000, Wei Liu wrote:
> On Wed, Mar 23, 2016 at 01:24:07PM +0100, Juergen Gross wrote:
> > Today the device model (qemu) is started for a pv domain only in case
> > a device requiring qemu is specified in the domain configuration
> > (qdisk, vfb, channel). If there is no such device the device model
> > isn't started and hence it is impossible to add such a device to the
> > domain later.
> > 
> > Add a domain configuration parameter to specify the device model is
> > to be started in any case. This will enable adding devices with a
> > qemu based backend later.
> > 
> > While the optimal solution would be to start the device model
> > automatically when needed this would require some major rework of
> > libxl at multiple places.
> > 
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> 
> I can't say I like this.
> 
> Libxl can be made smart enough to detect whether starting a device model
> is required (as you already did in previous patch) then start qemu on
> demand.
> 
> There is a xen-attach mode in qemu, and libxl already has machinery for
> starting qemu. I suspect what you need to do is refactoring existing
> code and expose the right parameter for qemu. Then you wouldn't need
> this user visible option at all. You would also save the resources for
> always having an idle qemu in toolstack domain.
> 
> Let me know if this is to vague.
> 

Actually I don't think this is your problem. I would consider this a bug
in libxl for not starting qemu on demand. I can take a stab at it, but
that will need to wait until next release.

I think the functionality you care about  can live without this patch,
but I'm not sure if it would become too cumbersome to use.

Wei.

> Wei.

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