Re: A comment about changing kernels

[Espen Skoglund]

[Jonathan S Shapiro]
> On Mon, 2005-10-31 at 19:06 +0100, Espen Skoglund wrote:
>> Just to set the record straight.  I believe that your comments here
>> are absolutely off target because:
>> 
>> a) His kernel supported small spaces.  There's no way to implement
>> small spaces correctly without reloading segment registers.  In
>> other words: your statement is completely wrong.
>> 
>> b) It makes absoultely no sense be paranoid about restricting the
>> communication channel resulting from reloading segment registers
>> when doing an IPC between two partners (i.e., you already have
>> the communication channel).  Avoiding such a communication
>> channel is only an issue when some other type of context switch
>> occurs (and this is not on the critical IPC path).
>> 
>> I know that you stated the same claims in your "Vulnerabilities..."
>> paper, and so these claims are now on record as being the "truth".

> Espen:

> It really was a bug, and it has nothing to do with the IPC path, and
> it has nothing to do with small spaces, so my statement may or may
> not be wrong. Let me describe the actual bug:

> [...]

> 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.  Additionally, if you look at Jochen's
numbers (the 135 cycles number you refer to) the slides he used for
presenting these numbers explicitly state that so and so many cycles
were spent reloading segment registers.

> Definitely not an earth-shattering issue, [...]

Perhaps so.  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.

I'm not meaning to attack you personally here.  I'm just saying that
many (most) of the statements you make I must take with a huge grain
of salt because I cannot trust them to be correct.  This lack of trust
is probably not in your interest, and I would therefore sincerly hope
that you try to be more careful about your choice of words.

        eSk