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

From: Jing Zhang <jingzhangos@google.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: KVM <kvm@vger.kernel.org>, KVMARM <kvmarm@lists.cs.columbia.edu>,
	LinuxMIPS <linux-mips@vger.kernel.org>,
	KVMPPC <kvm-ppc@vger.kernel.org>,
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	Paolo Bonzini <pbonzini@redhat.com>,
	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 17:15:57 -0500	[thread overview]
Message-ID: <CAAdAUthDbVvzFMGEp5sWEZ0MrtWaJBV4F_N4ois1D+4NO3n7fg@mail.gmail.com> (raw)
In-Reply-To: <CAAdAUtik0Yw+4=4Ld-DJSokNzvdpa-tsxkqAdBCAb-C=uS0-sA@mail.gmail.com>

Hi Greg,

On Thu, Jun 17, 2021 at 10:20 AM Jing Zhang <jingzhangos@google.com> wrote:
>
> Hi Greg,
>
> On Thu, Jun 17, 2021 at 12:56 AM Greg KH <gregkh@linuxfoundation.org> 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?  And then, really it is "used", so why not just say
> > that now?  It's tricky, but you have to get this right now otherwise you
> > can never use it in the future.
> >
> Sure, will do that.
> > > +The ``name`` field points to the name string of the statistics data. The name
> >
> > It is not a pointer, it is the data itself.
> >
> Will fix it.
> > > +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).
> >
> The reason to declare it as [0] is to have the flexibility to change the maximum
> length of KVM stats name. For now, the max len is  defined as 48, which can
> be read from the header. Then the userspace can get the length of descriptor by
> adding sizeof(struct_kvm_stats_desc) + 48. Whenever the max len is changed
> in KVM, the userspace would not have to update code to reflect that.
> However, if we are OK to restrict the maximum KVM stats' length to 48
> (or any other
> number), we can just declear it with [] instead of [0].
Please ignore my above comments.
You are right. We can just replace all zero-length arrays [0] with a
flexible array
member []. Thanks.
> > thanks,
> >
> > greg k-h
>
> Thanks,
> Jing