linux-kernel - Re: [PATCH 1/1] Fix int1 recursion when no perf_bp

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Date:	Thu, 10 Dec 2015 13:09:18 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Jeff Merkey <linux.mdb@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH 1/1] Fix int1 recursion when no perf_bp_event is registeredy

On Thu, Dec 10, 2015 at 12:49 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 10 Dec 2015, Andy Lutomirski wrote:
>> On brief inspection, this smells like a microcode bug.  Can you send
>> /proc/cpuinfo output?
>>
>> If this is the issue, I'm not sure we want to be in the business of
>> working around localized microcode bugs and, if we do, then I think we
>> should explicitly detect the bug and log about it.
>
> I think we should handle such stuff gracefully. Yes, we should log it
> and we also should check what the contents of the debug registers are.
>
> If dr7 has a break point enabled w/o perf having one installed then we
> know that someone did a horrible hackery ....

I mis-read this.  I don't think this is the microcode bug.

Do we know what the actual problem is? Jeff, how did you trigger this?

If it's lazy DR switching (which I haven't looked at the details of),
then it seems that this could just be triggered by some unfortunate
combination of perf config and context switching).  But, if so, then I
think that the proposed fix is wrong -- shouldn't we fix dr7 rather
than fudging RF to work around this instance of the problem?  After
all, if we're hitting this condition in a tight userspace loop, we're
going to destroy performance unless we fix dr7.

--Andy
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