Re: [PATCH 2/2] libxl: do not automatically force detach of block devices

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Sep 08, 2020 at 02:19:03PM +0000, Wei Liu wrote:
> On Thu, Sep 03, 2020 at 11:05:37AM +0100, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> > 
> > The manpage for 'xl' documents that guest co-operation is required for a 
> > (non-
> > forced) block-detach operation and that it may consequently fail. Currently,
> > however, the implementation of generic device removal means that a time-out
> > of a block-detach is being automatically re-tried with the force flag set
> > rather than failing. This patch stops such behaviour.
> > 
> > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> 
> I'm two-minded here.
> 
> On the one hand, special-casing VBD in libxl to conform to xl's
> behaviour seems wrong to me.
> 
> On the other hand, if we want to treat all devices the same in libxl,
> libxl should drop its force-removal behaviour all together, and the
> retry behaviour would need to be implemented in xl for all devices
> excepts for VBD. This requires a bit of code churn and, more
> importantly, changes how those device removal APIs behave. In the end
> this effort may not worth it.

I would be worried about those changes, as we would likely have to
also change libvirt or any other downstreams?

> If we go with the patch here, we should document this special case on
> libxl level somehow.
> 
> Anthony and Ian, do you have any opinion on this?

Maybe a new function should be introduced instead, that attempts to
remove a device gracefully and fail otherwise?

Then none of the current APIs would change, and xl could use this new
function to handle VBD removal?

Roger.