Re: [PATCH for-4.15] xen/dmop: Fix XEN_DMOP_nr_vcpus to actually return data

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Andrew Cooper writes ("[PATCH for-4.15] xen/dmop: Fix XEN_DMOP_nr_vcpus to 
actually return data"):
> The const_op boolean needs clobbering to cause data to be written back to the
> caller.
> 
> Fixes: c4441ab1f1 ("dmop: Add XEN_DMOP_nr_vcpus")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> CC: Ian Jackson <iwj@xxxxxxxxxxxxxx>
> 
> If we weren't in a release freeze, I'd rewrite large chunks of this.
> 'const_op' is what we call 'copyback' everywhere else, but with inverted
> sense.  I'll guess this gets added to the pile of other unbreakage work which
> might happen in 4.16
> 
> My ad-hoc unit test appears to have had a false positive for the success case,
> which I've fixed.  However, the chances of the full test landing in 4.15 is
> getting slimmer, not to mention the fact that it curretly takes out Xen with
> reference counting error...
> 
> As for 4.15, this is a bug in a brand-newly introduced hypercall, and is of 0
> risk for other areas of the release.  If this bugfix is not taken, we should
> revert c4441ab1f1 to take the hypercall out, but this would be a bad move.

Thanks for that explanation.

Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>