linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Message-ID: <20150507122913.GA17443@gmail.com>
Date:	Thu, 7 May 2015 14:29:13 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> > > We cannot take the lock_trace(task) from irq context, and we 
> > > probably do not need to anyway, since we do not care about a 
> > > precise stack trace for the task.
> > 
> > So one worry with this and similar approaches of statistically 
> > detecting user mode would be the fact that on the way out to 
> > user-space we don't really destroy the previous call trace - we 
> > just pop off the stack (non-destructively), restore RIPs and are 
> > gone.
> > 
> > We'll need that percpu flag I suspect.
> 
> Note we have the context tracking state which tells where the 
> current task is: user/system/guest.

Yes, but that overhead is what I'm suggesting we get rid of, I thought 
Rik was trying to find a mechanism that would be independent of that?

Thanks,

	Ingo
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