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Date: Mon, 21 Apr 2008 13:59:27 -0400
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Mathieu Desnoyers <compudj@krystal.dyndns.org>
CC: Pavel Machek <pavel@ucw.cz>, mingo@elte.hu, akpm@osdl.org,
       Jeremy Fitzhardinge <jeremy@goop.org>,
       Steven Rostedt <rostedt@goodmis.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] x86 NMI-safe INT3 and Page Fault (v5)
References: <20080417165839.GA25198@Krystal> <20080417165944.GB25198@Krystal> <20080417201410.GB31616@Krystal> <20080421140054.GB4685@ucw.cz> <480CA337.3090709@zytor.com> <20080421150825.GA4070@Krystal> <480CADD3.3010209@zytor.com> <20080421154756.GA2424@Krystal> <20080421172327.GA32278@elf.ucw.cz> <480CCEB0.8040401@zytor.com> <20080421174239.GA14718@Krystal>
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Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@zytor.com) wrote:
>> Pavel Machek wrote:
>>>> There is also the page fault case. I think putting this test in
>>>> ret_from_exception would be both safe (it is executed for any
>>>> exception return) and fast (exceptions are rare).
>>> Eh? I thought that page fault is one of the hottest paths in kernel
>>> (along with syscall and packet receive/send)...
>>> 								Pavel
> 
> On x86_64, we can pinpoint only the page faults returning to the kernel,
> which are rare and only caused by vmalloc accesses. Ideally we could do
> the same on x86_32.
> 

Pinpoint, how?  Ultimately you need a runtime test, and you better be 
showing that people are going to die unless before you add a cycle to 
the page fault path.  I'm only slightly exaggerating that.

	-hpa