[MODERATED] Re: [PATCH v5 00/27] MDSv5 19

[Andi Kleen <ak@linux.intel.com>]

> > everyone understands these implications.
> 
> No, we don't (or at least I do not), understand any of the implications
> here because the questions I asked were never answered.  So far you have

The original patches flushed on every kernel exit.
This was rejected as default because it has some overhead
(we saw ~6% for a micro benchmark, RH saw more in a very unrealistic
micro) 

The second (actually third) version didn't flush on every
kernel exit, but only on asynchronous events like non whitelisted
interrupts/timers/tasklets

The vast majority of the performance improvement came
from not flushing for process context system calls and exceptions.
At least in our benchmarks there was very little overhead
with that solution, with most kernel exits not flushing
anymore, and any penalties near the noise level.

But it relied on asynchronous events white listing
to avoid having to patch most of the tree.

The feedback, including from you, was that this would 
be still unacceptable to slow down any interrupt handlers or timers
that don't touch user data.

So the requirement was that all interrupt handlers/timers/
tasklets in the tree need to be audited, to only flush for the ones
that touch user data. That's what we've been
working on for the next version.

Obviously it's neither simple nor soon nor quick 
or easy to maintain.

> just "told us what you are going to do" and then never even backed that
> up with reasons for why you are doing that.

!?!? I wrote long document with a full security model.

Did you read it? I don't remember any comments on that from you.

> Why would anyone want to take
> that?

Are you asking about the mds=full variant, (like v2 of the patch series)

It's really simple, straight forward, and has a very strictly defined
easy security model that can be explained in a few sentences.

It's also very similar to what will be eventually in tree
as the code path for mds=full, there haven't been any changes
to this at all.

Its only drawback is some performance penalty. Everything else
is vastly superior over any other solutions.

If anyone just wants a simple and soon fix it's the obvious
choice.


-Andi