linux-kernel - Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption

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Date:   Thu, 4 Jul 2019 20:16:54 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        He Zhe <zhe.he@...driver.com>,
        Joel Fernandes <joel@...lfernandes.org>, devel@...ukata.com
Subject: Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption



> On Jul 4, 2019, at 7:18 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Fri, Jul 5, 2019 at 5:03 AM Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>> Despire the current efforts to read CR2 before tracing happens there
>> still exist a number of possible holes:
> 
> So this whole series disturbs me for the simple reason that I thought
> tracing was supposed to save/restore cr2 and make it unnecessary to
> worry about this in non-tracing code.
> 
> That is very much what the NMI code explicitly does. Why shouldn't all
> the other tracing code do the same thing in case they can take page
> faults?
> 

If nothing else, MOV to CR2 is architecturally serializing, so, unless there’s some fancy unwinding involved, this will be quite slow.