Re: 2.5isms

[Arjan van de Ven <arjan@infradead.org>]

> 
> I'm curious about a couple of points though. First, is that it is basically
> just adding a cache colouring to the stack, right? In that case why do only
> older HT CPUs have bad performance without it? And wouldn't it possibly make
> even non HT CPUs possibly slightly more efficient WRT caching the stacks of
> multiple processes?

it's a win on more than older HT cpus. It's just that those suffer it
the most... (since there you have 2 "cpus" share the cache, meaning you
get double the aliasing)


> Second, on what workloads does performance suffer, can you remember? I wonder
> if natural variations in the stack pointer as the program runs would mitigate
> the effect of this on all but micro benchmarks?

one of the problem cases I remember is network daemons all waiting in
accept() for connections. All from the same codepath basically.
Randomizing the stackpointer is a gain for that on all cpus that have
finite affinity on their caches.


> But even if that were so so, it seems simple enough that I don't have any
> real problem with keeping it of course.

The reason my patch does it much more is that it makes it a step harder
to write exploits for stack buffer overflows.