Re: [PATCH v10 3/5] KVM: stats: Add documentation for binary statistics interface

[Jing Zhang <jingzhangos@google.com>]

On Thu, Jun 17, 2021 at 6:32 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 17/06/21 07:56, Greg KH wrote:
> > On Thu, Jun 17, 2021 at 04:41:44AM +0000, Jing Zhang wrote:
> >> +    struct kvm_stats_desc {
> >> +            __u32 flags;
> >> +            __s16 exponent;
> >> +            __u16 size;
> >> +            __u32 offset;
> >> +            __u32 unused;
> >> +            char name[0];
> >> +    };
> >
> > <snip>
> >
> >> +The ``unused`` fields are reserved for future support for other types of
> >> +statistics data, like log/linear histogram.
> >
> > you HAVE to set unused to 0 for now, otherwise userspace does not know
> > it is unused, right?
>
> Jing, I think you planned to use it with other flags that are unused for
> now?  But please do check that it's zero in the testcase.
>
Yes, it was planned for future use (to support stats type of histogram).
Will add check in testcase and clarify it in doc.
> > It is not a pointer, it is the data itself.
> >
> >> +string starts at the end of ``struct kvm_stats_desc``.
> >> +The maximum length (including trailing '\0') is indicated by ``name_size``
> >> +in ``struct kvm_stats_header``.
> >
> > I thought we were replacing [0] arrays with [], are you sure you should
> > be declaring this as [0]?  Same for all structures in this document (and
> > code).
>
> In C code [0] is a bit more flexible than [].  I think in this
> particular case [] won't work due to how the structures are declared.
> In the documentation [] is certainly clearer.
>
> Paolo
>

Thanks,
Jing