Re: 2.5isms

[Nick Piggin]

Arjan van de Ven wrote:
> > 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.

I see. Yes, that would be a prime candidate.

> > 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. 

Oh yeah I realised that. I just meant specifically the code to do arch
specific stack colouring.

Thanks
Nick
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