Re: [for-5.0 PATCH] ppc: Make PPCVirtualHypervisor an incomplete type

[Philippe Mathieu-Daudé]

On 12/9/19 2:28 PM, Greg Kurz wrote:
> PPCVirtualHypervisor is an interface instance. It should never be
> dereferenced. Drop the dummy type definition for extra safety, which
> is the common practice with QOM interfaces.

This "common practice" is also referenced in commit 00ed3da9b5:

    xics: Minor fixes for XICSFabric interface

    Interface instances should never be directly dereferenced.  So, the 
common
    practice is to make them incomplete types to make sure no-one does 
that.
    XICSFrabric, however, had a dummy type which is less safe.

    We were also using OBJECT_CHECK() where we should have been using
    INTERFACE_CHECK().

This indeed follow the changes from commit aa1b35b975d8:

    qom: make interface types abstract

    Interfaces don't have instance, let's make the interface type really
    abstract to avoid confusion.

Now I can't find guidelines for this. If you don't know about it and use 
'git-grep', it is very confusing to see we use structures we never define.

Can we document this use please?

> Signed-off-by: Greg Kurz <address@hidden>
> ---
>   target/ppc/cpu.h |    4 ----
>   1 file changed, 4 deletions(-)
>
> diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
> index e3e82327b723..ab7d07d66047 100644
> --- a/target/ppc/cpu.h
> +++ b/target/ppc/cpu.h
> @@ -1220,10 +1220,6 @@ PowerPCCPUClass *ppc_cpu_class_by_pvr(uint32_t pvr);
>   PowerPCCPUClass *ppc_cpu_class_by_pvr_mask(uint32_t pvr);
>   PowerPCCPUClass *ppc_cpu_get_family_class(PowerPCCPUClass *pcc);
>   
> -struct PPCVirtualHypervisor {
> -    Object parent;
> -};
> -
>   struct PPCVirtualHypervisorClass {
>       InterfaceClass parent;
>       void (*hypercall)(PPCVirtualHypervisor *vhyp, PowerPCCPU *cpu);