linux-kernel - Re: perf_fuzzer compiled for x32 causes reboot

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Date:	Sat, 1 Mar 2014 10:50:42 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: perf_fuzzer compiled for x32 causes reboot

On Sat, Mar 01, 2014 at 10:16:50AM +0100, Ingo Molnar wrote:
> 
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > Also, this function is called a _LOT_ under certain workloads, I 
> > > don't know how cheap a CR2 read is, but it had better be really 
> > > cheap.
> > 
> > That's a HPA question.
> 
> We read CR2 in the page fault hot path, so it's on the top of CPU 
> architects' minds and it's reasonably optimized. A couple of cycles 
> IIRC, but would be nice to hear actual numbers.

Yeah, we were discussing this last night on IRC.

And hpa actually meant that the optimization potential was there but no
one did do it, except maybe Transmeta. :-)

So the expensive thing is writing to CR2 because it is a serializing
instruction. In fact, all writes to control registers except CR8 are
serializing.

The reading from CR2 should be cheaper but not as cheap as a normal
MOV %reg %reg is. On AMD, MOV %reg, %cr2 is done with microcode so
definitely at least a couple of cycles and I'd guess it is not a trivial
MOV on Intel too.

Maybe a way to hide this cost is the OoO, as hpa suggested, depending on
how much parallelism that particular code region can offer (serializing
instructions close by).

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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