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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

Wei.

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