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Subject: Re: [PATCH v2 0/5] Statsfs: a new ram-based file sytem for Linux
 kernel statistics
To:     Paolo Bonzini <pbonzini@redhat.com>,
        David Rientjes <rientjes@google.com>
Cc:     Jim Mattson <jmattson@google.com>,
        Emanuele Giuseppe Esposito <eesposit@redhat.com>,
        Jonathan Adams <jwadams@google.com>,
        kvm list <kvm@vger.kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mips@vger.kernel.org,
        kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-s390@vger.kernel.org,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Stefan Raspl <raspl@linux.ibm.com>
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Date:   Tue, 5 May 2020 19:30:32 +0200
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Adding Stefan Raspl, who has done a lot of kvm_stat work in the past.

On 05.05.20 19:21, Paolo Bonzini wrote:
> On 05/05/20 19:07, David Rientjes wrote:
>>> I am totally in favor of having a binary format, but it should be
>>> introduced as a separate series on top of this one---and preferably by
>>> someone who has already put some thought into the problem (which
>>> Emanuele and I have not, beyond ensuring that the statsfs concept and
>>> API is flexible enough).
>>>
>> The concern is that once this series is merged then /sys/kernel/stats 
>> could be considered an ABI and there would be a reasonable expectation 
>> that it will remain stable, in so far as the stats that userspace is 
>> interested in are stable and not obsoleted.
>>
>> So is this a suggestion that the binary format becomes complementary to 
>> statsfs and provide a means for getting all stats from a single subsystem, 
>> or that this series gets converted to such a format before it is merged?
> 
> The binary format should be complementary.  The ASCII format should
> indeed be considered stable even though individual statistics would come
> and go.  It may make sense to allow disabling ASCII files via mount
> and/or Kconfig options; but either way, the binary format can and should
> be added on top.
> 
> I have not put any thought into what the binary format would look like
> and what its features would be.  For example these are but the first
> questions that come to mind:
> 
> * would it be possible to read/clear an arbitrary statistic with
> pread/pwrite, or do you have to read all of them?
> 
> * if userspace wants to read the schema just once and then read the
> statistics many times, how is it informed of schema changes?
> 
> * and of course the details of how the schema (names of stat and
> subsources) is encoded and what details it should include about the
> values (e.g. type or just signedness).
> 
> Another possibility is to query stats via BPF.  This could be a third
> way to access the stats, or it could be alternative to a binary format.
> 
> Paolo
>