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

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Message-ID: <20140228160638.3b045dbc@gandalf.local.home>
Date:	Fri, 28 Feb 2014 16:06:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"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 Fri, 28 Feb 2014 21:56:38 +0100
Peter Zijlstra <peterz@...radead.org> wrote:


> Like already said; _trace is an absolutely abysmal name. Also you
> _really_ don't want an unconditional CR2 write in there, that's just
> stupidly expensive.

But a read isn't. Which is why we only do a write if the copy caused a
page fault (which is already expensive).

The proposed patch will have:

	if (cr2 != read_cr2())
		write_cr2(cr2);

> 
> 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.

-- Steve
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