Re: A comment about changing kernels

[Espen Skoglund]

[Jonathan S Shapiro]
> On Wed, 2005-11-02 at 13:36 +0100, Espen Skoglund wrote:
>>> The correct behavior was to save the segment selectors, and this
>>> adds about 20 cycles to the path. This means that the reported
>>> small space performance figure of 135 cycles/IPC was actually low
>>> by about 15% relative to a correct implementation. So the
>>> situation was that the (incorrect) L4 path was compared to the
>>> (correct) EROS path to suggest equal performance, but a corrected
>>> L4 path would have been measurably slower.
>> 
>> As you say yourself, this has nothing to do with the IPC path.  And
>> as Bernhard pointed out, this is purely an interrupt handling
>> issue.
>> 
>> I've also said that implementing small spaces without reloading
>> segment registers on IPC is *not* possible.  Your statement above
>> is therefore a blatant lie.

> Espen:

> In fact, it is only a misunderstanding that has taken a while to
> sort out. The *critical* clarifying statement arrived only in your
> very latest email -- or at least, this is the first time that I have
> understood it. Knowing you, I assume that your words are the result
> of extreme frustration rather than considered intent.

Well, the words "blatant lie" might have been quite a bit too harsh.
"Probably a terrible misunderstanding" would have been more to the
point.  Reading through the mail again I also realize that I could
definitely have been more clear in describing the save/restore
vs. reload differences.

> All: I have had a chance to think through Espen's latest comments,
> and he is correct. There is no bug in the L4 IPC path, except
> possibly a documentation bug. I have been traveling, so I have not
> been able to check, but I do not *remember* the IPC specification
> documenting that segment selectors are restored to known values by
> the IPC operation. The rationale that justifies the current behavior
> is, in my opinion, obscure, but it is correct. Let me try to walk
> everyone through the discussion in order to confirm what has
> converged.

The documentation only says that the segment selectors may change at
any point in time.  This also includes the point where a thread
returns from IPC.  It's true that this could indeed be made more clear
in the documentation for IPC ABI.  Not that it really matters for
user-level programs though, since no program can (according to the
spec) rely on using the segment selectors for anything useful anyway.

> Espen implies that this decision was intentional. If so, it is
> puzzling that Jochen did not point this out when I described this
> issue to him in 2000. On the contrary, Jochen *agreed* that this was
> a bug. Because he agreed, I did not investigate the matter further.

Agreed.  That definitely *is* puzzling.


>> I'm only pointing this out because you have a tendency of
>> presenting your own interpretations and beliefs as "truth".  Sure,
>> in some cases you may simply be inaccurate, ill informed, or not
>> remember exactly what has previously been said, but in these cases
>> you should have the decency to clearly express so.

> Espen: it is our *jobs* as engineers to state the results of our
> analysis and their implications, even when it later proves that
> these are wrong. It is also our jobs to correct our mistakes after
> we come to understand them. To my knowledge, I have *never* failed
> to acknowledge such an error once it has become clear to me. Can you
> name a *single* instance in which this has not been true?
> Conversely, can you identify a single case in which any member of
> the L4 community has retracted erroneous statements about *my* work?
> None of us are perfect.

Sorry.  I wasn't trying to insinuate that you've failed to acknowledge
errors or mistakes in any way.  On the contrary, you've always gone to
great lengths to do so.  I was only trying to say that many of the
things you say often display an air of heavy confidence about them.
Nothing that there's anything wrong with that per se, and in most
cases you certainly have the experience and knoweldge to display that
level of confidence.

My problem is that I sometimes see cases where this confidence seems
to not be 100% justified.  The big confusion therefore arises about
how I should interpret the rest of your statements.  How to know which
statements to question, and how to know which statements to take as
facts---how to separate the actual facts from the strong beliefs if
you like.  Having very little experience in many of the fields you
talk about can make it really difficult to make this distinction.  I
hope you can appreciate my problem here.

Again, I was never attempting to attack your integrity or personality
in any way (altough the tone in my mail could certainly have given
that impression).  My intention was to try and make you aware that
such distinctions that you may take for granted does not always seem
all that clear in the ears of people who lack some of your experience
and knowledge about the matter at hand.

> I will assemble a web page on the EROS site clarifying the current
> discussion and the get Geoff Voelker to send out a note to the
> SIGOPS list about it. Not because you complained, but because it is
> the only possible professionally responsible action. If you can see
> a better way to reach the relevant audience, please do not hesitate
> to let me know.

I'm not sure if the whole issue warrants more attention than it has
already got.  It does afterall seem to be a mere misunderstanding.
>From my point of view it would be better to just let the matter rest
and move on to more pressing issues.

        eSk