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

From: Jing Zhang <jingzhangos@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Greg KH <gregkh@linuxfoundation.org>, KVM <kvm@vger.kernel.org>,
	KVMARM <kvmarm@lists.cs.columbia.edu>,
	LinuxMIPS <linux-mips@vger.kernel.org>,
	KVMPPC <kvm-ppc@vger.kernel.org>,
	LinuxS390 <linux-s390@vger.kernel.org>,
	Linuxkselftest <linux-kselftest@vger.kernel.org>,
	Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Will Deacon <will@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Paul Mackerras <paulus@ozlabs.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.com>,
	Peter Shier <pshier@google.com>, Oliver Upton <oupton@google.com>,
	David Rientjes <rientjes@google.com>,
	Emanuele Giuseppe Esposito <eesposit@redhat.com>,
	David Matlack <dmatlack@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Krish Sadhukhan <krish.sadhukhan@oracle.com>,
	Fuad Tabba <tabba@google.com>
Subject: Re: [PATCH v10 3/5] KVM: stats: Add documentation for binary statistics interface
Date: Thu, 17 Jun 2021 10:45:10 -0500	[thread overview]
Message-ID: <CAAdAUtiAEp-+MydpamzysT4aAXvu9tvhOY0YecnQkGkWp6pJWA@mail.gmail.com> (raw)
In-Reply-To: <0d959828-da89-bceb-f7cc-35622a60c431@redhat.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