Re: [Xen-devel] [PATCH v2 4/5] libxl: change xs path for pv qemu

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Jun 08, 2015 at 04:27:26PM +0100, Stefano Stabellini wrote:
> On Mon, 8 Jun 2015, Wei Liu wrote:
> > On Thu, Jun 04, 2015 at 12:28:18PM +0100, Stefano Stabellini wrote:
> > > If QEMU is ran just to provide PV backends, change the xenstore path to
> > > /local/domain/0/device-model/$DOMID/pv.
> > > 
> > > Add a parameter to libxl__device_model_xs_path to distinguish the device
> > > model from the pv backends provider.
> > > 
> > > Store the device model binary path under
> > > /local/domain/$DOMID/device-model on xenstore, so that we can fetch it
> > > later and retrieve the list of supported options from
> > > /local/domain/0/libxl/$device_model_binary, introduce in the previous
> > > path.
> > > 
> > 
> > TBH this protocol works, but it is not very extensible.
> > 
> > I envisaged we need to assign $emulator_id to different device models
> > when I fixed stubdom, but I never got to that since there weren't need
> > for multiple emulators.
> > 
> > That is, as an example:
> > 
> > /local/domain/$backend_domid/device-model/$domid/$emulator_id/xxx
> > 
> > That way we can:
> > 
> > 1. Have something like multidev in libxl to wait for several device
> >    models to be ready without writing tedious code for every single one.
> > 2. Fit into libxl migration stream, which naturally uses $emulator_id to
> >    distinguish different emulators. 
> > 
> > The downside of this is we need to add an extra option to QEMU to accept
> > the emulator id assigned by toolstack.
> > 
> > Just my two cents.
> 
> Actually QEMU already retrieves the ioservid by calling
> xc_hvm_create_ioreq_server. I could easily use that as id. I am not sure
> how I could get that from libxl though, except by assuming that is going
> to be 0, because libxl only knows how to create one QEMU emulator.
> 

I don't think it's good to use that id directly because as you said
there is no way to allocate and get that from libxl. Libxl needs to
arrange xenstore directory before QEMU starts up, i.e. before QEMU is
able to allocate such id from hypervisor.

Assigning an emulator id would be easy in libxl.

> As there is no ioservid for pv QEMU, I could still use
> /local/domain/$backend_domid/device-model/$domid/pv.
> 
> What do you think?
> 

Do you envisage we would need to use several PV qemu? At that point we
would face the same problem again. We might as well solve it now.

Wei.

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